Re: Warning on mremapped uffd-wp memory

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On 07.08.24 10:58, David Hildenbrand wrote:

On 06.08.24 22:29, Peter Xu wrote:

On Tue, Aug 06, 2024 at 06:37:55PM +0200, David Hildenbrand wrote:

On 06.08.24 17:15, Ryan Roberts wrote:

Hi Peter, David,


Hi, Ryan,



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:


This is true.  I can easily reproduce..



[ 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.

Is the vma flag maybe getting dropped because of some weird interaction with
UFFD_EVENT_REMAP?


Right, I think we should do the latter.

We need to drop the vma flag by default, as you quoted in the other patch
in 2018, as the monitor process may not be able to process this otherwise,
seeing unknown address reported when read(). So instead we should drop the
uffd-wp bit here..


For the records: I don't think the patch from 2018 made the right call.

Was there a particular reason for the VMA flag changes (BUG report?). I
can see why the wrongly sent event was problematic and had to be fixed.

When dropping these VMA flags, especially for the missing mode, the app
will suddenly get *wrong* data. Instead of the uffd monitor being in
charge what to place, we will give it zeroed pages.

To me that translates to a silent memory corruption.

In contrast, if the monitor the pagefault information and let him
realize that most likely he should be using UFFD_EVENT_REMAP.

I would reconsider that change in 2018. To me it would make more sense
to not drop flags during mremap.
... looking into the implementation some more, I think there might be more issues lurking, and I'm not 100% sure what the right semantics are.
Assume we use mremap() to grow a VMA that has uffd registered. Looks like (did not try reproducing) vma_expandable() would just work and we would end up growing the uffd range by doing vma_merge_extend(). Not sure if that's intended or what the expected semantics are at all with all different corner cases of mremap. 


--
Cheers,

David / dhildenb
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