Re: [PATCH] mm/vma: fix anon_vma UAF on mremap() faulted, unfaulted merge

["David Hildenbrand (Red Hat)" <david@kernel.org>]

On 1/2/26 21:55, Lorenzo Stoakes wrote:
> Commit 879bca0a2c4f ("mm/vma: fix incorrectly disallowed anonymous VMA
> merges") introduced the ability to merge previously unavailable VMA merge
> scenarios.
> 
> The key piece of logic introduced was the ability to merge a faulted VMA
> immediately next to an unfaulted VMA, which relies upon dup_anon_vma() to
> correctly handle anon_vma state.
> 
> In the case of the merge of an existing VMA (that is changing properties of
> a VMA and then merging if those properties are shared by adjacent VMAs),
> dup_anon_vma() is invoked correctly.
> 
> However in the case of the merge of a new VMA, a corner case peculiar to
> mremap() was missed.
> 
> The issue is that vma_expand() only performs dup_anon_vma() if the target
> (the VMA that will ultimately become the merged VMA): is not the next VMA,
> i.e. the one that appears after the range in which the new VMA is to be
> established.
> 
> A key insight here is that in all other cases other than mremap(), a new
> VMA merge either expands an existing VMA, meaning that the target VMA will
> be that VMA, or would have anon_vma be NULL.
> 
> Specifically:
> 
> * __mmap_region() - no anon_vma in place, initial mapping.
> * do_brk_flags() - expanding an existing VMA.
> * vma_merge_extend() - expanding an existing VMA.
> * relocate_vma_down() - no anon_vma in place, initial mapping.
> 
> In addition, we are in the unique situation of needing to duplicate
> anon_vma state from a VMA that is neither the previous or next VMA being
> merged with.
> 
> To account for this, introduce a new field in struct vma_merge_struct
> specifically for the mremap() case, and update vma_expand() to explicitly
> check for this case and invoke dup_anon_vma() to ensure anon_vma state is
> correctly propagated.
> 
> This issue can be observed most directly by invoked mremap() to move around
> a VMA and cause this kind of merge with the MREMAP_DONTUNMAP flag
> specified.
> 
> This will result in unlink_anon_vmas() being called after failing to
> duplicate anon_vma state to the target VMA, which results in the anon_vma
> itself being freed with folios still possessing dangling pointers to the
> anon_vma and thus a use-after-free bug.

Makes sense to me.

> 
> This bug was discovered via a syzbot report, which this patch resolves.
> 
> The following program reproduces the issue (and is fixed by this patch):
> 
> #define _GNU_SOURCE
> #include <stdio.h>
> #include <stdlib.h>
> #include <unistd.h>
> #include <sys/mman.h>
> 
> #define RESERVED_PGS	(100)
> #define VMA_A_PGS	(10)
> #define VMA_B_PGS	(10)
> #define NUM_ITERS	(1000)
> 
> static void trigger_bug(void)
> {
> 	unsigned long page_size = sysconf(_SC_PAGE_SIZE);
> 	char *reserved, *ptr_a, *ptr_b;
> 
> 	/*
> 	 * The goal here is to achieve:
> 	 *
> 	 * mremap() with MREMAP_DONTUNMAP such that A and B merge:
> 	 *
> 	 *      |-------------------------|
> 	 *      |                         |
> 	 *      |    |-----------|   |---------|
> 	 *      v    | unfaulted |   | faulted |
> 	 *           |-----------|   |---------|
> 	 *                 B              A
> 	 *
> 	 * Then unmap VMA A to trigger the bug.
> 	 */
> 
> 	/* Reserve a region of memory to operate in. */
> 	reserved = mmap(NULL, RESERVED_PGS * page_size, PROT_NONE,
> 			MAP_PRIVATE | MAP_ANON, -1, 0);
> 	if (reserved == MAP_FAILED) {
> 		perror("mmap reserved");
> 		exit(EXIT_FAILURE);
> 	}
> 
> 	/* Map VMA A into place. */
> 	ptr_a = mmap(&reserved[page_size], VMA_A_PGS * page_size,
> 		     PROT_READ | PROT_WRITE,
> 		     MAP_PRIVATE | MAP_ANON | MAP_FIXED, -1, 0);
> 	if (ptr_a == MAP_FAILED) {
> 		perror("mmap VMA A");
> 		exit(EXIT_FAILURE);
> 	}
> 	/* Fault it in. */
> 	ptr_a[0] = 'x';
> 
> 	/*
> 	 * Now move it out of the way so we can place VMA B in position,
> 	 * unfaulted.
> 	 */
> 	ptr_a = mremap(ptr_a, VMA_A_PGS * page_size, VMA_A_PGS * page_size,
> 		       MREMAP_FIXED | MREMAP_MAYMOVE, &reserved[50 * page_size]);
> 	if (ptr_a == MAP_FAILED) {
> 		perror("mremap VMA A out of the way");
> 		exit(EXIT_FAILURE);
> 	}
> 
> 	/* Map VMA B into place. */
> 	ptr_b = mmap(&reserved[page_size + VMA_A_PGS * page_size],
> 		     VMA_B_PGS * page_size, PROT_READ | PROT_WRITE,
> 		     MAP_PRIVATE | MAP_ANON | MAP_FIXED, -1, 0);
> 	if (ptr_b == MAP_FAILED) {
> 		perror("mmap VMA B");
> 		exit(EXIT_FAILURE);
> 	}
> 
> 	/* Now move VMA A into position w/MREMAP_DONTUNMAP + free anon_vma. */
> 	ptr_a = mremap(ptr_a, VMA_A_PGS * page_size, VMA_A_PGS * page_size,
> 		       MREMAP_FIXED | MREMAP_MAYMOVE | MREMAP_DONTUNMAP,
> 		       &reserved[page_size]);
> 	if (ptr_a == MAP_FAILED) {
> 		perror("mremap VMA A with MREMAP_DONTUNMAP");
> 		exit(EXIT_FAILURE);
> 	}
> 
> 	/* Finally, unmap VMA A which should trigger the bug. */
> 	munmap(ptr_a, VMA_A_PGS * page_size);
> 
> 	/* Cleanup in case bug didn't trigger sufficiently visibly... */
> 	munmap(reserved, RESERVED_PGS * page_size);
> }
> 
> int main(void)
> {
> 	int i;
> 
> 	for (i = 0; i < NUM_ITERS; i++)
> 		trigger_bug();

Just wondering, why do we have to loop, I would have thought that this would
trigger deterministically.

> 
> 	return EXIT_SUCCESS;
> }
> 
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> Fixes: 879bca0a2c4f ("mm/vma: fix incorrectly disallowed anonymous VMA merges")
> Reported-by: syzbot+b165fc2e11771c66d8ba@syzkaller.appspotmail.com
> Closes: https://lore.kernel.org/all/694a2745.050a0220.19928e.0017.GAE@google.com/
> Cc: stable@kernel.org

I was wondering whether this commit actually fixes older reports that Jann
mentioned in his commit a222439e1e27 ("mm/rmap: add anon_vma lifetime debug check").

     [1] https://lore.kernel.org/r/67abaeaf.050a0220.110943.0041.GAE@google.com
     [2] https://lore.kernel.org/r/67a76f33.050a0220.3d72c.0028.GAE@google.com


But 879bca0a2c4f went into v6.16, but [1] and [2] are against v6.14.

So naturally I wonder, could it be that we had a bug even before 879bca0a2c4f that
resulted in similar symptoms?

Option (1): [1] and [2] are already fixed

Option (2): [1] and [2] are still broken

Option (3): [1] and [2] would be fixed by your patch as well

But we don't even have reproducers, so [1] and [2] could just be a side-effect of
another bug, maybe.



> ---
>   mm/vma.c | 58 ++++++++++++++++++++++++++++++++++++++++++--------------
>   mm/vma.h |  3 +++
>   2 files changed, 47 insertions(+), 14 deletions(-)
> 
> diff --git a/mm/vma.c b/mm/vma.c
> index 6377aa290a27..2268f518a89b 100644
> --- a/mm/vma.c
> +++ b/mm/vma.c
> @@ -1130,26 +1130,50 @@ int vma_expand(struct vma_merge_struct *vmg)
>   	mmap_assert_write_locked(vmg->mm);
> 
>   	vma_start_write(target);
> -	if (next && (target != next) && (vmg->end == next->vm_end)) {
> +	if (next && vmg->end == next->vm_end) {
> +		struct vm_area_struct *copied_from = vmg->copied_from;
>   		int ret;
> 
> -		sticky_flags |= next->vm_flags & VM_STICKY;
> -		remove_next = true;
> -		/* This should already have been checked by this point. */
> -		VM_WARN_ON_VMG(!can_merge_remove_vma(next), vmg);
> -		vma_start_write(next);
> -		/*
> -		 * In this case we don't report OOM, so vmg->give_up_on_mm is
> -		 * safe.
> -		 */
> -		ret = dup_anon_vma(target, next, &anon_dup);
> -		if (ret)
> -			return ret;
> +		if (target != next) {
> +			sticky_flags |= next->vm_flags & VM_STICKY;
> +			remove_next = true;
> +			/* This should already have been checked by this point. */
> +			VM_WARN_ON_VMG(!can_merge_remove_vma(next), vmg);
> +			vma_start_write(next);
> +			/*
> +			 * In this case we don't report OOM, so vmg->give_up_on_mm is
> +			 * safe.
> +			 */
> +			ret = dup_anon_vma(target, next, &anon_dup);
> +			if (ret)
> +				return ret;
> +		} else if (copied_from) {
> +			vma_start_write(next);
> +
> +			/*
> +			 * We are copying from a VMA (i.e. mremap()'ing) to
> +			 * next, and thus must ensure that either anon_vma's are
> +			 * already compatible (in which case this call is a nop)
> +			 * or all anon_vma state is propagated to next
> +			 */
> +			ret = dup_anon_vma(next, copied_from, &anon_dup);
> +			if (ret)
> +				return ret;
> +		} else {
> +			/* In no other case may the anon_vma differ. */
> +			VM_WARN_ON_VMG(target->anon_vma != next->anon_vma, vmg);
> +		}


No expert on that code, but looks reasonable to me.

Wondering whether we want to pull the  vma_start_write(next) before the loop
(for the warn we certainly don't care).

-- 
Cheers

David