pti.rst (9119B)
1.. SPDX-License-Identifier: GPL-2.0 2 3========================== 4Page Table Isolation (PTI) 5========================== 6 7Overview 8======== 9 10Page Table Isolation (pti, previously known as KAISER [1]_) is a 11countermeasure against attacks on the shared user/kernel address 12space such as the "Meltdown" approach [2]_. 13 14To mitigate this class of attacks, we create an independent set of 15page tables for use only when running userspace applications. When 16the kernel is entered via syscalls, interrupts or exceptions, the 17page tables are switched to the full "kernel" copy. When the system 18switches back to user mode, the user copy is used again. 19 20The userspace page tables contain only a minimal amount of kernel 21data: only what is needed to enter/exit the kernel such as the 22entry/exit functions themselves and the interrupt descriptor table 23(IDT). There are a few strictly unnecessary things that get mapped 24such as the first C function when entering an interrupt (see 25comments in pti.c). 26 27This approach helps to ensure that side-channel attacks leveraging 28the paging structures do not function when PTI is enabled. It can be 29enabled by setting CONFIG_PAGE_TABLE_ISOLATION=y at compile time. 30Once enabled at compile-time, it can be disabled at boot with the 31'nopti' or 'pti=' kernel parameters (see kernel-parameters.txt). 32 33Page Table Management 34===================== 35 36When PTI is enabled, the kernel manages two sets of page tables. 37The first set is very similar to the single set which is present in 38kernels without PTI. This includes a complete mapping of userspace 39that the kernel can use for things like copy_to_user(). 40 41Although _complete_, the user portion of the kernel page tables is 42crippled by setting the NX bit in the top level. This ensures 43that any missed kernel->user CR3 switch will immediately crash 44userspace upon executing its first instruction. 45 46The userspace page tables map only the kernel data needed to enter 47and exit the kernel. This data is entirely contained in the 'struct 48cpu_entry_area' structure which is placed in the fixmap which gives 49each CPU's copy of the area a compile-time-fixed virtual address. 50 51For new userspace mappings, the kernel makes the entries in its 52page tables like normal. The only difference is when the kernel 53makes entries in the top (PGD) level. In addition to setting the 54entry in the main kernel PGD, a copy of the entry is made in the 55userspace page tables' PGD. 56 57This sharing at the PGD level also inherently shares all the lower 58layers of the page tables. This leaves a single, shared set of 59userspace page tables to manage. One PTE to lock, one set of 60accessed bits, dirty bits, etc... 61 62Overhead 63======== 64 65Protection against side-channel attacks is important. But, 66this protection comes at a cost: 67 681. Increased Memory Use 69 70 a. Each process now needs an order-1 PGD instead of order-0. 71 (Consumes an additional 4k per process). 72 b. The 'cpu_entry_area' structure must be 2MB in size and 2MB 73 aligned so that it can be mapped by setting a single PMD 74 entry. This consumes nearly 2MB of RAM once the kernel 75 is decompressed, but no space in the kernel image itself. 76 772. Runtime Cost 78 79 a. CR3 manipulation to switch between the page table copies 80 must be done at interrupt, syscall, and exception entry 81 and exit (it can be skipped when the kernel is interrupted, 82 though.) Moves to CR3 are on the order of a hundred 83 cycles, and are required at every entry and exit. 84 b. A "trampoline" must be used for SYSCALL entry. This 85 trampoline depends on a smaller set of resources than the 86 non-PTI SYSCALL entry code, so requires mapping fewer 87 things into the userspace page tables. The downside is 88 that stacks must be switched at entry time. 89 c. Global pages are disabled for all kernel structures not 90 mapped into both kernel and userspace page tables. This 91 feature of the MMU allows different processes to share TLB 92 entries mapping the kernel. Losing the feature means more 93 TLB misses after a context switch. The actual loss of 94 performance is very small, however, never exceeding 1%. 95 d. Process Context IDentifiers (PCID) is a CPU feature that 96 allows us to skip flushing the entire TLB when switching page 97 tables by setting a special bit in CR3 when the page tables 98 are changed. This makes switching the page tables (at context 99 switch, or kernel entry/exit) cheaper. But, on systems with 100 PCID support, the context switch code must flush both the user 101 and kernel entries out of the TLB. The user PCID TLB flush is 102 deferred until the exit to userspace, minimizing the cost. 103 See intel.com/sdm for the gory PCID/INVPCID details. 104 e. The userspace page tables must be populated for each new 105 process. Even without PTI, the shared kernel mappings 106 are created by copying top-level (PGD) entries into each 107 new process. But, with PTI, there are now *two* kernel 108 mappings: one in the kernel page tables that maps everything 109 and one for the entry/exit structures. At fork(), we need to 110 copy both. 111 f. In addition to the fork()-time copying, there must also 112 be an update to the userspace PGD any time a set_pgd() is done 113 on a PGD used to map userspace. This ensures that the kernel 114 and userspace copies always map the same userspace 115 memory. 116 g. On systems without PCID support, each CR3 write flushes 117 the entire TLB. That means that each syscall, interrupt 118 or exception flushes the TLB. 119 h. INVPCID is a TLB-flushing instruction which allows flushing 120 of TLB entries for non-current PCIDs. Some systems support 121 PCIDs, but do not support INVPCID. On these systems, addresses 122 can only be flushed from the TLB for the current PCID. When 123 flushing a kernel address, we need to flush all PCIDs, so a 124 single kernel address flush will require a TLB-flushing CR3 125 write upon the next use of every PCID. 126 127Possible Future Work 128==================== 1291. We can be more careful about not actually writing to CR3 130 unless its value is actually changed. 1312. Allow PTI to be enabled/disabled at runtime in addition to the 132 boot-time switching. 133 134Testing 135======== 136 137To test stability of PTI, the following test procedure is recommended, 138ideally doing all of these in parallel: 139 1401. Set CONFIG_DEBUG_ENTRY=y 1412. Run several copies of all of the tools/testing/selftests/x86/ tests 142 (excluding MPX and protection_keys) in a loop on multiple CPUs for 143 several minutes. These tests frequently uncover corner cases in the 144 kernel entry code. In general, old kernels might cause these tests 145 themselves to crash, but they should never crash the kernel. 1463. Run the 'perf' tool in a mode (top or record) that generates many 147 frequent performance monitoring non-maskable interrupts (see "NMI" 148 in /proc/interrupts). This exercises the NMI entry/exit code which 149 is known to trigger bugs in code paths that did not expect to be 150 interrupted, including nested NMIs. Using "-c" boosts the rate of 151 NMIs, and using two -c with separate counters encourages nested NMIs 152 and less deterministic behavior. 153 :: 154 155 while true; do perf record -c 10000 -e instructions,cycles -a sleep 10; done 156 1574. Launch a KVM virtual machine. 1585. Run 32-bit binaries on systems supporting the SYSCALL instruction. 159 This has been a lightly-tested code path and needs extra scrutiny. 160 161Debugging 162========= 163 164Bugs in PTI cause a few different signatures of crashes 165that are worth noting here. 166 167 * Failures of the selftests/x86 code. Usually a bug in one of the 168 more obscure corners of entry_64.S 169 * Crashes in early boot, especially around CPU bringup. Bugs 170 in the trampoline code or mappings cause these. 171 * Crashes at the first interrupt. Caused by bugs in entry_64.S, 172 like screwing up a page table switch. Also caused by 173 incorrectly mapping the IRQ handler entry code. 174 * Crashes at the first NMI. The NMI code is separate from main 175 interrupt handlers and can have bugs that do not affect 176 normal interrupts. Also caused by incorrectly mapping NMI 177 code. NMIs that interrupt the entry code must be very 178 careful and can be the cause of crashes that show up when 179 running perf. 180 * Kernel crashes at the first exit to userspace. entry_64.S 181 bugs, or failing to map some of the exit code. 182 * Crashes at first interrupt that interrupts userspace. The paths 183 in entry_64.S that return to userspace are sometimes separate 184 from the ones that return to the kernel. 185 * Double faults: overflowing the kernel stack because of page 186 faults upon page faults. Caused by touching non-pti-mapped 187 data in the entry code, or forgetting to switch to kernel 188 CR3 before calling into C functions which are not pti-mapped. 189 * Userspace segfaults early in boot, sometimes manifesting 190 as mount(8) failing to mount the rootfs. These have 191 tended to be TLB invalidation issues. Usually invalidating 192 the wrong PCID, or otherwise missing an invalidation. 193 194.. [1] https://gruss.cc/files/kaiser.pdf 195.. [2] https://meltdownattack.com/meltdown.pdf