mm: correctly reference merged VMA

Commit Message

Lorenzo Stoakes Dec. 6, 2024, 9:52 p.m. UTC

On second merge attempt on mmap() we incorrectly discard the possibly
merged VMA, resulting in a possible use-after-free (and most certainly a
reference to the wrong VMA) in this instance in the subsequent
__mmap_complete() invocation.

Correct this mistake by reassigning vma correctly if a merge succeeds in
this case.

Suggested-by: Jann Horn <[e-mail]>
Reported-by: [e-mail]
Closes: https://lore.kernel.org/all/[e-mail]/
Fixes: 5ac87a885aec ("mm: defer second attempt at merge on mmap()")
Signed-off-by: Lorenzo Stoakes <[e-mail]>
---
 mm/vma.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

--
2.47.1

Comments

Lorenzo Stoakes Dec. 6, 2024, 9:55 p.m. UTC | #1

Andrew - sorry, I forgot to put a subject-prefix in here - we need this as
a (fairly urgent) hotfix for 6.13 please.

Thanks!

On Fri, Dec 06, 2024 at 09:52:29PM +0000, Lorenzo Stoakes wrote:
> On second merge attempt on mmap() we incorrectly discard the possibly
> merged VMA, resulting in a possible use-after-free (and most certainly a
> reference to the wrong VMA) in this instance in the subsequent
> __mmap_complete() invocation.
>
> Correct this mistake by reassigning vma correctly if a merge succeeds in
> this case.
>
> Suggested-by: Jann Horn <[e-mail]>
> Reported-by: [e-mail]
> Closes: https://lore.kernel.org/all/[e-mail]/
> Fixes: 5ac87a885aec ("mm: defer second attempt at merge on mmap()")
> Signed-off-by: Lorenzo Stoakes <[e-mail]>
> ---
>  mm/vma.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/mm/vma.c b/mm/vma.c
> index 8a454a7bbc80..1ec349141f5e 100644
> --- a/mm/vma.c
> +++ b/mm/vma.c
> @@ -2460,10 +2460,13 @@ unsigned long __mmap_region(struct file *file, unsigned long addr,
>
>  	/* If flags changed, we might be able to merge, so try again. */
>  	if (map.retry_merge) {
> +		struct vm_area_struct *merged;
>  		VMG_MMAP_STATE(vmg, &map, vma);
>
>  		vma_iter_config(map.vmi, map.addr, map.end);
> -		vma_merge_existing_range(&vmg);
> +		merged = vma_merge_existing_range(&vmg);
> +		if (merged)
> +			vma = merged;
>  	}
>
>  	__mmap_complete(&map, vma);
> --
> 2.47.1

Liam R. Howlett Dec. 6, 2024, 10:40 p.m. UTC | #2

* Lorenzo Stoakes <[e-mail]> [241206 16:52]:
> On second merge attempt on mmap() we incorrectly discard the possibly
> merged VMA, resulting in a possible use-after-free (and most certainly a
> reference to the wrong VMA) in this instance in the subsequent
> __mmap_complete() invocation.
> 
> Correct this mistake by reassigning vma correctly if a merge succeeds in
> this case.
> 
> Suggested-by: Jann Horn <[e-mail]>
> Reported-by: [e-mail]
> Closes: https://lore.kernel.org/all/[e-mail]/
> Fixes: 5ac87a885aec ("mm: defer second attempt at merge on mmap()")
> Signed-off-by: Lorenzo Stoakes <[e-mail]>

My cries to delete this optimisation continue.

Reviewed-by: Liam R. Howlett <[e-mail]>

> ---
>  mm/vma.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/vma.c b/mm/vma.c
> index 8a454a7bbc80..1ec349141f5e 100644
> --- a/mm/vma.c
> +++ b/mm/vma.c
> @@ -2460,10 +2460,13 @@ unsigned long __mmap_region(struct file *file, unsigned long addr,
> 
>  	/* If flags changed, we might be able to merge, so try again. */
>  	if (map.retry_merge) {
> +		struct vm_area_struct *merged;
>  		VMG_MMAP_STATE(vmg, &map, vma);
> 
>  		vma_iter_config(map.vmi, map.addr, map.end);
> -		vma_merge_existing_range(&vmg);
> +		merged = vma_merge_existing_range(&vmg);
> +		if (merged)
> +			vma = merged;
>  	}
> 
>  	__mmap_complete(&map, vma);
> --
> 2.47.1

Lorenzo Stoakes Dec. 9, 2024, 11:02 a.m. UTC | #3

On Fri, Dec 06, 2024 at 05:40:39PM -0500, Liam R. Howlett wrote:
> * Lorenzo Stoakes <[e-mail]> [241206 16:52]:
> > On second merge attempt on mmap() we incorrectly discard the possibly
> > merged VMA, resulting in a possible use-after-free (and most certainly a
> > reference to the wrong VMA) in this instance in the subsequent
> > __mmap_complete() invocation.
> >
> > Correct this mistake by reassigning vma correctly if a merge succeeds in
> > this case.
> >
> > Suggested-by: Jann Horn <[e-mail]>
> > Reported-by: [e-mail]
> > Closes: https://lore.kernel.org/all/[e-mail]/
> > Fixes: 5ac87a885aec ("mm: defer second attempt at merge on mmap()")
> > Signed-off-by: Lorenzo Stoakes <[e-mail]>
>
> My cries to delete this optimisation continue.

I'd love to too, but it does seem that has an important use in that you get a
file system/driver that sets certain VMA flags.

So you have:

         [  VMA B  ] <- map
[ VMA A ] < space > [ VMA C ]

Merge attempt #1 - missing VM_xxx that is set by f_op->mmap() hook
Merge attempt #2 - succeed because VM_xxx is set.

This, I think, speaks to the need to provide a means by which
filesystems/drivers can manipulate VMA flags _up front_. At which point we can
eliminate this.

I mean as you say this is an -optimisation- and having split VMAs will work fine
(modulo mremap weirdness), but because of the above it becomes possible for
there to be a major proliferation of unneeded VMAs, so it's not quite as much of
a fringe thing as it might appear to be.

I mean we can also talk about the fact that there's logic in __mmap_complete()
that clears VM_LOCKED | VM_LOCKONFAULT under certain circumstances which is just
not considered for merging at all but... yeah. That is, at least, much less
likely.

>
> Reviewed-by: Liam R. Howlett <[e-mail]>

Thanks :)

>
> > ---
> >  mm/vma.c | 5 ++++-
> >  1 file changed, 4 insertions(+), 1 deletion(-)
> >
> > diff --git a/mm/vma.c b/mm/vma.c
> > index 8a454a7bbc80..1ec349141f5e 100644
> > --- a/mm/vma.c
> > +++ b/mm/vma.c
> > @@ -2460,10 +2460,13 @@ unsigned long __mmap_region(struct file *file, unsigned long addr,
> >
> >  	/* If flags changed, we might be able to merge, so try again. */
> >  	if (map.retry_merge) {
> > +		struct vm_area_struct *merged;
> >  		VMG_MMAP_STATE(vmg, &map, vma);
> >
> >  		vma_iter_config(map.vmi, map.addr, map.end);
> > -		vma_merge_existing_range(&vmg);
> > +		merged = vma_merge_existing_range(&vmg);
> > +		if (merged)
> > +			vma = merged;
> >  	}
> >
> >  	__mmap_complete(&map, vma);
> > --
> > 2.47.1

Vlastimil Babka Dec. 9, 2024, 1:19 p.m. UTC | #4

On 12/6/24 22:52, Lorenzo Stoakes wrote:
> On second merge attempt on mmap() we incorrectly discard the possibly
> merged VMA, resulting in a possible use-after-free (and most certainly a
> reference to the wrong VMA) in this instance in the subsequent
> __mmap_complete() invocation.
> 
> Correct this mistake by reassigning vma correctly if a merge succeeds in
> this case.
> 
> Suggested-by: Jann Horn <[e-mail]>
> Reported-by: [e-mail]
> Closes: https://lore.kernel.org/all/[e-mail]/
> Fixes: 5ac87a885aec ("mm: defer second attempt at merge on mmap()")

FFS, who reviewed that patch?

> Signed-off-by: Lorenzo Stoakes <[e-mail]>

Reviewed-by: Vlastimil Babka <[e-mail]>

(copy/pasted from "git show 5ac87a885aec")

> ---
>  mm/vma.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/vma.c b/mm/vma.c
> index 8a454a7bbc80..1ec349141f5e 100644
> --- a/mm/vma.c
> +++ b/mm/vma.c
> @@ -2460,10 +2460,13 @@ unsigned long __mmap_region(struct file *file, unsigned long addr,
> 
>  	/* If flags changed, we might be able to merge, so try again. */
>  	if (map.retry_merge) {
> +		struct vm_area_struct *merged;
>  		VMG_MMAP_STATE(vmg, &map, vma);
> 
>  		vma_iter_config(map.vmi, map.addr, map.end);
> -		vma_merge_existing_range(&vmg);
> +		merged = vma_merge_existing_range(&vmg);
> +		if (merged)
> +			vma = merged;
>  	}
> 
>  	__mmap_complete(&map, vma);
> --
> 2.47.1

Patch

diff --git a/mm/vma.c b/mm/vma.c
index 8a454a7bbc80..1ec349141f5e 100644
--- a/mm/vma.c
+++ b/mm/vma.c
@@ -2460,10 +2460,13 @@  unsigned long __mmap_region(struct file *file, unsigned long addr,

 	/* If flags changed, we might be able to merge, so try again. */
 	if (map.retry_merge) {
+		struct vm_area_struct *merged;
 		VMG_MMAP_STATE(vmg, &map, vma);

 		vma_iter_config(map.vmi, map.addr, map.end);
-		vma_merge_existing_range(&vmg);
+		merged = vma_merge_existing_range(&vmg);
+		if (merged)
+			vma = merged;
 	}

 	__mmap_complete(&map, vma);