196 lines
8.9 KiB
ReStructuredText
196 lines
8.9 KiB
ReStructuredText
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.. SPDX-License-Identifier: GPL-2.0
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==========================
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Page Table Isolation (PTI)
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==========================
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Overview
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========
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Page Table Isolation (pti, previously known as KAISER [1]_) is a
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countermeasure against attacks on the shared user/kernel address
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space such as the "Meltdown" approach [2]_.
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To mitigate this class of attacks, we create an independent set of
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page tables for use only when running userspace applications. When
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the kernel is entered via syscalls, interrupts or exceptions, the
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page tables are switched to the full "kernel" copy. When the system
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switches back to user mode, the user copy is used again.
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The userspace page tables contain only a minimal amount of kernel
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data: only what is needed to enter/exit the kernel such as the
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entry/exit functions themselves and the interrupt descriptor table
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(IDT). There are a few strictly unnecessary things that get mapped
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such as the first C function when entering an interrupt (see
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comments in pti.c).
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This approach helps to ensure that side-channel attacks leveraging
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the paging structures do not function when PTI is enabled. It can be
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enabled by setting CONFIG_PAGE_TABLE_ISOLATION=y at compile time.
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Once enabled at compile-time, it can be disabled at boot with the
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'nopti' or 'pti=' kernel parameters (see kernel-parameters.txt).
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Page Table Management
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=====================
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When PTI is enabled, the kernel manages two sets of page tables.
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The first set is very similar to the single set which is present in
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kernels without PTI. This includes a complete mapping of userspace
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that the kernel can use for things like copy_to_user().
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Although _complete_, the user portion of the kernel page tables is
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crippled by setting the NX bit in the top level. This ensures
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that any missed kernel->user CR3 switch will immediately crash
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userspace upon executing its first instruction.
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The userspace page tables map only the kernel data needed to enter
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and exit the kernel. This data is entirely contained in the 'struct
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cpu_entry_area' structure which is placed in the fixmap which gives
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each CPU's copy of the area a compile-time-fixed virtual address.
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For new userspace mappings, the kernel makes the entries in its
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page tables like normal. The only difference is when the kernel
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makes entries in the top (PGD) level. In addition to setting the
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entry in the main kernel PGD, a copy of the entry is made in the
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userspace page tables' PGD.
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This sharing at the PGD level also inherently shares all the lower
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layers of the page tables. This leaves a single, shared set of
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userspace page tables to manage. One PTE to lock, one set of
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accessed bits, dirty bits, etc...
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Overhead
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========
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Protection against side-channel attacks is important. But,
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this protection comes at a cost:
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1. Increased Memory Use
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a. Each process now needs an order-1 PGD instead of order-0.
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(Consumes an additional 4k per process).
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b. The 'cpu_entry_area' structure must be 2MB in size and 2MB
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aligned so that it can be mapped by setting a single PMD
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entry. This consumes nearly 2MB of RAM once the kernel
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is decompressed, but no space in the kernel image itself.
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2. Runtime Cost
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a. CR3 manipulation to switch between the page table copies
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must be done at interrupt, syscall, and exception entry
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and exit (it can be skipped when the kernel is interrupted,
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though.) Moves to CR3 are on the order of a hundred
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cycles, and are required at every entry and exit.
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b. A "trampoline" must be used for SYSCALL entry. This
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trampoline depends on a smaller set of resources than the
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non-PTI SYSCALL entry code, so requires mapping fewer
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things into the userspace page tables. The downside is
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that stacks must be switched at entry time.
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c. Global pages are disabled for all kernel structures not
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mapped into both kernel and userspace page tables. This
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feature of the MMU allows different processes to share TLB
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entries mapping the kernel. Losing the feature means more
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TLB misses after a context switch. The actual loss of
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performance is very small, however, never exceeding 1%.
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d. Process Context IDentifiers (PCID) is a CPU feature that
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allows us to skip flushing the entire TLB when switching page
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tables by setting a special bit in CR3 when the page tables
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are changed. This makes switching the page tables (at context
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switch, or kernel entry/exit) cheaper. But, on systems with
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PCID support, the context switch code must flush both the user
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and kernel entries out of the TLB. The user PCID TLB flush is
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deferred until the exit to userspace, minimizing the cost.
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See intel.com/sdm for the gory PCID/INVPCID details.
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e. The userspace page tables must be populated for each new
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process. Even without PTI, the shared kernel mappings
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are created by copying top-level (PGD) entries into each
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new process. But, with PTI, there are now *two* kernel
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mappings: one in the kernel page tables that maps everything
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and one for the entry/exit structures. At fork(), we need to
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copy both.
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f. In addition to the fork()-time copying, there must also
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be an update to the userspace PGD any time a set_pgd() is done
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on a PGD used to map userspace. This ensures that the kernel
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and userspace copies always map the same userspace
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memory.
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g. On systems without PCID support, each CR3 write flushes
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the entire TLB. That means that each syscall, interrupt
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or exception flushes the TLB.
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h. INVPCID is a TLB-flushing instruction which allows flushing
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of TLB entries for non-current PCIDs. Some systems support
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PCIDs, but do not support INVPCID. On these systems, addresses
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can only be flushed from the TLB for the current PCID. When
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flushing a kernel address, we need to flush all PCIDs, so a
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single kernel address flush will require a TLB-flushing CR3
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write upon the next use of every PCID.
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Possible Future Work
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====================
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1. We can be more careful about not actually writing to CR3
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unless its value is actually changed.
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2. Allow PTI to be enabled/disabled at runtime in addition to the
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boot-time switching.
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Testing
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========
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To test stability of PTI, the following test procedure is recommended,
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ideally doing all of these in parallel:
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1. Set CONFIG_DEBUG_ENTRY=y
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2. Run several copies of all of the tools/testing/selftests/x86/ tests
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(excluding MPX and protection_keys) in a loop on multiple CPUs for
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several minutes. These tests frequently uncover corner cases in the
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kernel entry code. In general, old kernels might cause these tests
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themselves to crash, but they should never crash the kernel.
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3. Run the 'perf' tool in a mode (top or record) that generates many
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frequent performance monitoring non-maskable interrupts (see "NMI"
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in /proc/interrupts). This exercises the NMI entry/exit code which
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is known to trigger bugs in code paths that did not expect to be
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interrupted, including nested NMIs. Using "-c" boosts the rate of
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NMIs, and using two -c with separate counters encourages nested NMIs
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and less deterministic behavior.
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::
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while true; do perf record -c 10000 -e instructions,cycles -a sleep 10; done
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4. Launch a KVM virtual machine.
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5. Run 32-bit binaries on systems supporting the SYSCALL instruction.
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This has been a lightly-tested code path and needs extra scrutiny.
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Debugging
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=========
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Bugs in PTI cause a few different signatures of crashes
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that are worth noting here.
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* Failures of the selftests/x86 code. Usually a bug in one of the
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more obscure corners of entry_64.S
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* Crashes in early boot, especially around CPU bringup. Bugs
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in the trampoline code or mappings cause these.
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* Crashes at the first interrupt. Caused by bugs in entry_64.S,
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like screwing up a page table switch. Also caused by
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incorrectly mapping the IRQ handler entry code.
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* Crashes at the first NMI. The NMI code is separate from main
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interrupt handlers and can have bugs that do not affect
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normal interrupts. Also caused by incorrectly mapping NMI
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code. NMIs that interrupt the entry code must be very
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careful and can be the cause of crashes that show up when
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running perf.
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* Kernel crashes at the first exit to userspace. entry_64.S
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bugs, or failing to map some of the exit code.
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* Crashes at first interrupt that interrupts userspace. The paths
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in entry_64.S that return to userspace are sometimes separate
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from the ones that return to the kernel.
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* Double faults: overflowing the kernel stack because of page
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faults upon page faults. Caused by touching non-pti-mapped
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data in the entry code, or forgetting to switch to kernel
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CR3 before calling into C functions which are not pti-mapped.
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* Userspace segfaults early in boot, sometimes manifesting
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as mount(8) failing to mount the rootfs. These have
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tended to be TLB invalidation issues. Usually invalidating
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the wrong PCID, or otherwise missing an invalidation.
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.. [1] https://gruss.cc/files/kaiser.pdf
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.. [2] https://meltdownattack.com/meltdown.pdf
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