Re: [PATCH v2 04/40] mm: consistently validate VMA state after mmap[_prepare] hooks

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 4 potential issue(s) to consider:

New issues:
- [High] Premature execution of vma_close() before unmap_region() during validation failure creates a Use-After-Free race window.
- [High] Restoring vma->vm_start before calling unmap_region() on the validation error path leaves orphaned PTEs, causing permanent UAF.

Pre-existing issues:
- [High] mmap_validate() fails to check if a driver erroneously sets VMA_WRITE_BIT on a mapping without VMA_MAYWRITE_BIT, allowing read-only files to be mapped writable.
- [High] mmap_validate() fails to check if a driver alters vma->vm_end, which silently corrupts the maple tree.
--

commit d07b85544000595b057fc1ff43289af0359a7b6c
Author: Lorenzo Stoakes (ARM) <ljs@kernel.org>

mm: consistently validate VMA state after mmap[_prepare] hooks

This patch generalizes VMA state validation checks for both mmap_prepare
and legacy mmap hooks. It aims to ensure that drivers do not violate
invariants, such as altering the VMA start address or creating writable
mappings on read-only files, and adds validations for insert_vm_struct().

> diff --git a/mm/internal.h b/mm/internal.h
> index fe576d468af45..1ed11eaf276a7 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
[ ... ]
> @@ -239,26 +260,16 @@ static inline int mmap_file(struct file *file, struct vm_area_struct *vma)
>  	 */
>  	if (unlikely(err || !vma->vm_ops))
>  		vma->vm_ops = &vma_dummy_vm_ops;
> +	if (unlikely(err))
> +		return err;
>  
> -	return err;
> -}
> -
> -/*
> - * If the VMA has a close hook then close it, and since closing it might leave
> - * it in an inconsistent state which makes the use of any hooks suspect, clear
> - * them down by installing dummy empty hooks.
> - */
> -static inline void vma_close(struct vm_area_struct *vma)
> -{
> -	if (vma->vm_ops && vma->vm_ops->close) {
> -		vma->vm_ops->close(vma);
> -
> -		/*
> -		 * The mapping is in an inconsistent state, and no further hooks
> -		 * may be invoked upon it.
> -		 */
> -		vma->vm_ops = &vma_dummy_vm_ops;
> +	err = mmap_hook_validate(prev_start, &prev_flags, vma);
> +	if (unlikely(err)) {
> +		vma->vm_start = prev_start;

[Severity: High]
If a driver's legacy mmap hook erroneously decreases vma->vm_start (expanding
the VMA downwards) and populates PTEs in that expanded range,
mmap_hook_validate() catches the violation. By forcing vma->vm_start back to
prev_start here before __mmap_new_file_vma() invokes unmap_region(), doesn't
that initialize the unmap operation with the smaller, restored address range?
This would cause unmap_region() to skip the expanded PTEs, leaving them
orphaned and causing a permanent use-after-free since the backing memory gets
freed. Should the unmap occur before restoring the start address?

> +		vma_close(vma);

[Severity: High]
Is it safe to invoke vma_close() here immediately on validation failure?
If a driver successfully populates page tables (for example, via
remap_pfn_range) but fails the mmap_hook_validate() checks, calling
vma_close() frees driver memory prematurely. Since unmap_region() is not
called until mmap_file() returns to __mmap_new_file_vma(), hardware PTEs
remain populated while the backing resources are freed, creating a
use-after-free race window.

>  	}
> +
> +	return err;
>  }
>  
>  /* unmap_vmas is in mm/memory.c */
> diff --git a/mm/vma.c b/mm/vma.c
> index dc74b3d721cc5..626a18c08a269 100644
> --- a/mm/vma.c
> +++ b/mm/vma.c
[ ... ]
> @@ -2777,6 +2762,76 @@ static void __mmap_complete(struct mmap_state *map, struct vm_area_struct *vma)
>  	vma_set_page_prot(vma);
>  }
>  
> +/* Check to ensure that the VMA flags of a newly mapped VMA are sane. */
> +static int mmap_validate_vma_flags(const vma_flags_t *flags)
> +{
[ ... ]
> +/* Check to ensure a driver hasn't done something crazy. */
> +static int mmap_validate(unsigned long prev_start,
> +			 unsigned long curr_start,
> +			 const vma_flags_t *prev_flags,
> +			 const vma_flags_t *curr_flags)
> +{
> +	bool was_maywrite, is_maywrite;
> +
> +	/* Drivers cannot alter the address of the VMA. */
> +	if (WARN_ON_ONCE(prev_start != curr_start))
> +		return -EINVAL;

[Severity: High]
This isn't a bug introduced by this patch, but since the patch aims to
implement comprehensive VMA state validation after mmap hooks, shouldn't it
also verify that the end address (vma->vm_end) remains unmodified? If a
buggy driver alters vm_end, the VMA object's bounds will mismatch the maple
tree node's pre-allocated bounds. This structural inconsistency corrupts
the maple tree and can lead to kernel panics later.

> +
> +	was_maywrite = vma_flags_test(prev_flags, VMA_MAYWRITE_BIT);
> +	is_maywrite = vma_flags_test(curr_flags, VMA_MAYWRITE_BIT);
> +
> +	/* A driver may not make a previously unwritable mapping writable. */
> +	if (WARN_ON_ONCE(!was_maywrite && is_maywrite))
> +		return -EINVAL;

[Severity: High]
This is a pre-existing issue, but this check appears to bypass a critical
condition. It only evaluates VMA_MAYWRITE_BIT. If a malicious or buggy
driver bitwise-ORs VMA_WRITE_BIT into vma->flags in its mmap hook without
setting VMA_MAYWRITE_BIT, wouldn't it bypass this check completely? This
could allow a read-only file mapping to be committed with VM_WRITE set,
granting hardware write permissions and allowing an unprivileged user to
overwrite read-only files.

> +
> +	return mmap_validate_vma_flags(curr_flags);
> +}

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260914-b4-mmap-prepare-vma-flag-sanify-v2-0-7d9781ed5361@kernel.org?part=4