On 06/08/2024 17:37, David Hildenbrand wrote: > On 06.08.24 17:15, Ryan Roberts wrote: >> Hi Peter, David, >> >> syzkaller has found an issue (at least on arm64, but I suspect it will be >> visible on x86_64 too) that triggers the following warning: >> >> [ 2291.836518] ------------[ cut here ]------------ >> [ 2291.836528] WARNING: CPU: 3 PID: 9056 at mm/page_table_check.c:207 >> __page_table_check_ptes_set+0x22c/0x248 >> [ 2291.836541] Modules linked in: >> [ 2291.836549] CPU: 3 UID: 1000 PID: 9056 Comm: bug Tainted: G >> W 6.11.0-rc2-dirty #2 >> [ 2291.836554] Tainted: [W]=WARN >> [ 2291.836557] Hardware name: linux,dummy-virt (DT) >> [ 2291.836559] pstate: 80400005 (Nzcv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--) >> [ 2291.836564] pc : __page_table_check_ptes_set+0x22c/0x248 >> [ 2291.836568] lr : ptep_modify_prot_commit+0x24c/0x2b0 >> [ 2291.836573] sp : ffff80008ca6ba20 >> [ 2291.836575] x29: ffff80008ca6ba20 x28: ffff186392d1eb00 x27: 0000000020ffd000 >> [ 2291.836598] x26: 0010000000000001 x25: 0000000000000001 x24: 0000000000000000 >> [ 2291.836605] x23: 04e800018c738f43 x22: 0000000000000001 x21: ffff1863824163c0 >> [ 2291.836612] x20: 04e800018c738f43 x19: 04e800018c738f43 x18: 0000fffff7f87fff >> [ 2291.836619] x17: 0000000000000000 x16: 1fffe30c748d22a1 x15: 0060000000000fc3 >> [ 2291.836625] x14: 0000000000000000 x13: 0000000020ffd000 x12: 0000fffff7f87fff >> [ 2291.836631] x11: 0000000020ffd000 x10: 0000000000000000 x9 : ffffbcab99e3ab84 >> [ 2291.836638] x8 : ffff186382b8f000 x7 : 0000000020ffe000 x6 : 0000000020ffd000 >> [ 2291.836644] x5 : ffff186392d1eb00 x4 : 04e800018c738f43 x3 : 0000000000000001 >> [ 2291.836650] x2 : 04e800018c738f43 x1 : ffff18639fe01fe8 x0 : ffffbcab9ce56780 >> [ 2291.836657] Call trace: >> [ 2291.836659] __page_table_check_ptes_set+0x22c/0x248 >> [ 2291.836664] ptep_modify_prot_commit+0x24c/0x2b0 >> [ 2291.836667] change_protection+0x8a0/0x1100 >> [ 2291.836672] mprotect_fixup+0x124/0x2d0 >> [ 2291.836675] do_mprotect_pkey.constprop.0+0x29c/0x460 >> [ 2291.836679] __arm64_sys_mprotect+0x24/0xf8 >> [ 2291.836682] invoke_syscall+0x50/0x120 >> [ 2291.836690] el0_svc_common.constprop.0+0x48/0xf0 >> [ 2291.836694] do_el0_svc+0x24/0x38 >> [ 2291.836699] el0_svc+0x34/0xe0 >> [ 2291.836705] el0t_64_sync_handler+0x100/0x130 >> [ 2291.836709] el0t_64_sync+0x190/0x198 >> [ 2291.836713] ---[ end trace 0000000000000000 ]--- >> >> The generated program (see below) mmaps a 16M region (RWX). It then mlocks all >> current and future memory. >> >> Next, it registers 12K (3 pages) for use with UFFD-WP, and marks 4 pages >> UFFD-WP'ed. This returns ENOENT because we only registered 3 pages, but those 3 >> pages are still UFFD-WP'ed in their PTE, so this error is not relavent to the >> bug. At this point, there is a single VMA covering the 12K, with VM_UFFD_WP set, >> amongst other flags: >> >> 20ffb000-20ffe000 rwxp 00000000 00:00 0 >> Size: 12 kB >> KernelPageSize: 4 kB >> MMUPageSize: 4 kB >> Rss: 12 kB >> Pss: 12 kB >> Pss_Dirty: 12 kB >> Shared_Clean: 0 kB >> Shared_Dirty: 0 kB >> Private_Clean: 0 kB >> Private_Dirty: 12 kB >> Referenced: 12 kB >> Anonymous: 12 kB >> KSM: 0 kB >> LazyFree: 0 kB >> AnonHugePages: 0 kB >> ShmemPmdMapped: 0 kB >> FilePmdMapped: 0 kB >> Shared_Hugetlb: 0 kB >> Private_Hugetlb: 0 kB >> Swap: 0 kB >> SwapPss: 0 kB >> Locked: 12 kB >> THPeligible: 0 >> VmFlags: rd wr ex mr mw me uw lo ac >> >> Next we mremap the first page to the address where the last page was previously >> mapped, with MREMAP_DONTUNMAP. This leads to 2 VMAs, but the new one doesn't >> have VM_UFFD_WP set (Note also that the original VMA no longer has VM_LOCKED >> which seems wrong to me, but I'll ignore that for now): >> >> 20ffb000-20ffd000 rwxp 00000000 00:00 0 >> Size: 8 kB >> KernelPageSize: 4 kB >> MMUPageSize: 4 kB >> Rss: 4 kB >> Pss: 4 kB >> Pss_Dirty: 4 kB >> Shared_Clean: 0 kB >> Shared_Dirty: 0 kB >> Private_Clean: 0 kB >> Private_Dirty: 4 kB >> Referenced: 4 kB >> Anonymous: 4 kB >> KSM: 0 kB >> LazyFree: 0 kB >> AnonHugePages: 0 kB >> ShmemPmdMapped: 0 kB >> FilePmdMapped: 0 kB >> Shared_Hugetlb: 0 kB >> Private_Hugetlb: 0 kB >> Swap: 0 kB >> SwapPss: 0 kB >> Locked: 0 kB >> THPeligible: 0 >> VmFlags: rd wr ex mr mw me uw ac >> 20ffd000-20ffe000 rwxp 00000000 00:00 0 >> Size: 4 kB >> KernelPageSize: 4 kB >> MMUPageSize: 4 kB >> Rss: 4 kB >> Pss: 4 kB >> Pss_Dirty: 4 kB >> Shared_Clean: 0 kB >> Shared_Dirty: 0 kB >> Private_Clean: 0 kB >> Private_Dirty: 4 kB >> Referenced: 4 kB >> Anonymous: 4 kB >> KSM: 0 kB >> LazyFree: 0 kB >> AnonHugePages: 0 kB >> ShmemPmdMapped: 0 kB >> FilePmdMapped: 0 kB >> Shared_Hugetlb: 0 kB >> Private_Hugetlb: 0 kB >> Swap: 0 kB >> SwapPss: 0 kB >> Locked: 4 kB >> THPeligible: 0 >> VmFlags: rd wr ex mr mw me lo ac >> >> Finally we try to mprotect that last 4K region to remove X, and we get the >> warning saying the PTE has both the UFFD-WP and WRITE bits set. >> >> I'm guessing this is because the VM_UFFD_WP flag got spuriously dropped when >> creating the final 4K VMA and so mprotect's can_change_pte_writable() check >> incorrectly allowed the pte to be marked writable. But the mremap man page is >> not very clear on the semantics when interacting with uffd regions; perhaps >> uffd-wp bit should have been cleared when mremapping the ptes? >> >> I'm hoping you can advice on the expected semantics and we can figure out how to >> solve this? >> >> >> The reproducer is as follows (with a few annotations added by me): >> >> """ >> // autogenerated by syzkaller (https://github.com/google/syzkaller) >> >> #define _GNU_SOURCE >> >> #include <endian.h> >> #include <stdint.h> >> #include <stdio.h> >> #include <stdlib.h> >> #include <string.h> >> #include <sys/syscall.h> >> #include <sys/types.h> >> #include <unistd.h> >> >> #ifndef __NR_ioctl >> #define __NR_ioctl 29 >> #endif >> #ifndef __NR_mlockall >> #define __NR_mlockall 230 >> #endif >> #ifndef __NR_mmap >> #define __NR_mmap 222 >> #endif >> #ifndef __NR_mprotect >> #define __NR_mprotect 226 >> #endif >> #ifndef __NR_mremap >> #define __NR_mremap 216 >> #endif >> #ifndef __NR_userfaultfd >> #define __NR_userfaultfd 282 >> #endif >> >> uint64_t r[1] = {0xffffffffffffffff}; >> >> int main(void) >> { >> intptr_t res = 0; >> >> syscall(__NR_mmap, /*addr=*/0x1ffff000ul, /*len=*/0x1000ul, /*prot=*/0ul, >> /*flags=MAP_FIXED|MAP_ANONYMOUS|MAP_PRIVATE*/0x32ul, /*fd=*/-1, /*offset=*/0ul); >> syscall(__NR_mmap, /*addr=*/0x20000000ul, /*len=*/0x1000000ul, >> /*prot=PROT_WRITE|PROT_READ|PROT_EXEC*/7ul, >> /*flags=MAP_FIXED|MAP_ANONYMOUS|MAP_PRIVATE*/0x32ul, /*fd=*/-1, /*offset=*/0ul); >> syscall(__NR_mmap, /*addr=*/0x21000000ul, /*len=*/0x1000ul, /*prot=*/0ul, >> /*flags=MAP_FIXED|MAP_ANONYMOUS|MAP_PRIVATE*/0x32ul, /*fd=*/-1, /*offset=*/0ul); >> >> write(1, "executing program\n", sizeof("executing program\n") - 1); >> >> // userfaultfd(UFFD_USER_MODE_ONLY) = 3 >> res = syscall(__NR_userfaultfd, /*flags=UFFD_USER_MODE_ONLY*/1ul); >> if (res != -1) >> r[0] = res; >> >> // ioctl(3, UFFDIO_API, {api=0xaa, features=0 => >> features=UFFD_FEATURE_PAGEFAULT_FLAG_WP|UFFD_FEATURE_EVENT_FORK|UFFD_FEATURE_EVENT_REMAP|UFFD_FEATURE_EVENT_REMOVE|UFFD_FEATURE_MISSING_HUGETLBFS|UFFD_FEATURE_MISSING_SHMEM|UFFD_FEATURE_EVENT_UNMAP|UFFD_FEATURE_SIGBUS|UFFD_FEATURE_THREAD_ID|UFFD_FEATURE_MINOR_HUGETLBFS|UFFD_FEATURE_MINOR_SHMEM|0x1f800, ioctls=1<<_UFFDIO_REGISTER|1<<_UFFDIO_UNREGISTER|1<<_UFFDIO_API}) = 0 >> *(uint64_t*)0x20000000 = 0xaa; >> *(uint64_t*)0x20000008 = 0; >> *(uint64_t*)0x20000010 = 0; >> syscall(__NR_ioctl, /*fd=*/r[0], /*cmd=*/0xc018aa3f, /*arg=*/0x20000000ul); >> >> syscall(__NR_mlockall, /*flags=MCL_FUTURE|MCL_CURRENT*/3ul); >> >> // ioctl(3, UFFDIO_REGISTER, {range={start=0x20ffb000, len=0x3000}, >> mode=UFFDIO_REGISTER_MODE_WP, >> ioctls=1<<_UFFDIO_WAKE|1<<_UFFDIO_COPY|1<<_UFFDIO_ZEROPAGE|1<<_UFFDIO_WRITEPROTECT|0x120}) = 0 >> *(uint64_t*)0x20000180 = 0x20ffb000; >> *(uint64_t*)0x20000188 = 0x3000; >> *(uint64_t*)0x20000190 = 2; >> *(uint64_t*)0x20000198 = 0; >> syscall(__NR_ioctl, /*fd=*/r[0], /*cmd=*/0xc020aa00, /*arg=*/0x20000180ul); >> >> // ioctl(3, UFFDIO_WRITEPROTECT, 0x20000080) = -1 ENOENT (No such file or >> directory) >> *(uint64_t*)0x20000080 = 0x20ffb000; >> *(uint64_t*)0x20000088 = 0x4000; >> *(uint64_t*)0x20000090 = 1; >> syscall(__NR_ioctl, /*fd=*/r[0], /*cmd=*/0xc018aa06, /*arg=*/0x20000080ul); >> >> syscall(__NR_mremap, /*addr=*/0x20ffb000ul, /*len=*/0x1000ul, >> /*newlen=*/0x1000ul, >> /*flags=MREMAP_DONTUNMAP|MREMAP_FIXED|MREMAP_MAYMOVE*/7ul, >> /*newaddr=*/0x20ffd000ul); >> syscall(__NR_mprotect, /*addr=*/0x20ffd000ul, /*len=*/0x1000ul, >> /*prot=PROT_WRITE|PROT_READ*/3ul); >> >> return 0; >> } >> """ >> >> I'd appreciate any thoughts you may have! > > Interesting. Either the vma flag shouldn't get dropped or we should un-mark the > PTEs. Yes, agreed. But which? I guess Peter is the expert here? > > Is the vma flag maybe getting dropped because of some weird interaction with > UFFD_EVENT_REMAP? >
