[PATCH 1/1] mm/secretmem: fix use-after-free race in fault handler

[PATCH 1/1] mm/secretmem: fix use-after-free race in fault handler

[Lance Yang]

From: Lance Yang <lance.yang@linux.dev>

The error path in secretmem_fault() frees a folio before restoring its
direct map status, which is a race leading to a panic.

Fix the ordering to restore the map before the folio is freed.

Cc: <stable@vger.kernel.org>
Reported-by: Google Big Sleep <big-sleep-vuln-reports@google.com>
Closes: https://lore.kernel.org/linux-mm/CAEXGt5QeDpiHTu3K9tvjUTPqo+d-=wuCNYPa+6sWKrdQJ-ATdg@mail.gmail.com/
Signed-off-by: Lance Yang <lance.yang@linux.dev>
---
 mm/secretmem.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/secretmem.c b/mm/secretmem.c
index c1bd9a4b663d..37f6d1097853 100644
--- a/mm/secretmem.c
+++ b/mm/secretmem.c
@@ -82,13 +82,13 @@ static vm_fault_t secretmem_fault(struct vm_fault *vmf)
 		__folio_mark_uptodate(folio);
 		err = filemap_add_folio(mapping, folio, offset, gfp);
 		if (unlikely(err)) {
-			folio_put(folio);
 			/*
 			 * If a split of large page was required, it
 			 * already happened when we marked the page invalid
 			 * which guarantees that this call won't fail
 			 */
 			set_direct_map_default_noflush(folio_page(folio, 0));
+			folio_put(folio);
 			if (err == -EEXIST)
 				goto retry;
 
-- 
2.49.0

Re: [PATCH 1/1] mm/secretmem: fix use-after-free race in fault handler

[Mike Rapoport]

On Fri, Oct 31, 2025 at 05:18:18PM +0800, Lance Yang wrote:
> From: Lance Yang <lance.yang@linux.dev>
> 
> The error path in secretmem_fault() frees a folio before restoring its
> direct map status, which is a race leading to a panic.

Let's use the issue description from the report:

When a page fault occurs in a secret memory file created with
`memfd_secret(2)`, the kernel will allocate a new folio for it, mark
the underlying page as not-present in the direct map, and add it to
the file mapping.

If two tasks cause a fault in the same page concurrently, both could
end up allocating a folio and removing the page from the direct map,
but only one would succeed in adding the folio to the file
mapping. The task that failed undoes the effects of its attempt by (a)
freeing the folio again and (b) putting the page back into the direct
map. However, by doing these two operations in this order, the page
becomes available to the allocator again before it is placed back in
the direct mapping.

If another task attempts to allocate the page between (a) and (b), and
the kernel tries to access it via the direct map, it would result in a
supervisor not-present page fault.
 
> Fix the ordering to restore the map before the folio is freed.

... restore the direct map

With these changes

Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

> 
> Cc: <stable@vger.kernel.org>
> Reported-by: Google Big Sleep <big-sleep-vuln-reports@google.com>
> Closes: https://lore.kernel.org/linux-mm/CAEXGt5QeDpiHTu3K9tvjUTPqo+d-=wuCNYPa+6sWKrdQJ-ATdg@mail.gmail.com/
> Signed-off-by: Lance Yang <lance.yang@linux.dev>
> ---
>  mm/secretmem.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/secretmem.c b/mm/secretmem.c
> index c1bd9a4b663d..37f6d1097853 100644
> --- a/mm/secretmem.c
> +++ b/mm/secretmem.c
> @@ -82,13 +82,13 @@ static vm_fault_t secretmem_fault(struct vm_fault *vmf)
>  		__folio_mark_uptodate(folio);
>  		err = filemap_add_folio(mapping, folio, offset, gfp);
>  		if (unlikely(err)) {
> -			folio_put(folio);
>  			/*
>  			 * If a split of large page was required, it
>  			 * already happened when we marked the page invalid
>  			 * which guarantees that this call won't fail
>  			 */
>  			set_direct_map_default_noflush(folio_page(folio, 0));
> +			folio_put(folio);
>  			if (err == -EEXIST)
>  				goto retry;
>  
> -- 
> 2.49.0
> 

-- 
Sincerely yours,
Mike.

Re: [PATCH 1/1] mm/secretmem: fix use-after-free race in fault handler

[David Hildenbrand]

On 31.10.25 10:59, Mike Rapoport wrote:
> On Fri, Oct 31, 2025 at 05:18:18PM +0800, Lance Yang wrote:
>> From: Lance Yang <lance.yang@linux.dev>
>>
>> The error path in secretmem_fault() frees a folio before restoring its
>> direct map status, which is a race leading to a panic.
> 
> Let's use the issue description from the report:
> 
> When a page fault occurs in a secret memory file created with
> `memfd_secret(2)`, the kernel will allocate a new folio for it, mark
> the underlying page as not-present in the direct map, and add it to
> the file mapping.
> 
> If two tasks cause a fault in the same page concurrently, both could
> end up allocating a folio and removing the page from the direct map,
> but only one would succeed in adding the folio to the file
> mapping. The task that failed undoes the effects of its attempt by (a)
> freeing the folio again and (b) putting the page back into the direct
> map. However, by doing these two operations in this order, the page
> becomes available to the allocator again before it is placed back in
> the direct mapping.
> 
> If another task attempts to allocate the page between (a) and (b), and
> the kernel tries to access it via the direct map, it would result in a
> supervisor not-present page fault.
>   
>> Fix the ordering to restore the map before the folio is freed.
> 
> ... restore the direct map
> 
> With these changes
> 
> Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

Fully agreed

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Cheers

David / dhildenb

Re: [PATCH 1/1] mm/secretmem: fix use-after-free race in fault handler

[Lance Yang]

On 2025/10/31 18:19, David Hildenbrand wrote:
> On 31.10.25 10:59, Mike Rapoport wrote:
>> On Fri, Oct 31, 2025 at 05:18:18PM +0800, Lance Yang wrote:
>>> From: Lance Yang <lance.yang@linux.dev>
>>>
>>> The error path in secretmem_fault() frees a folio before restoring its
>>> direct map status, which is a race leading to a panic.
>>
>> Let's use the issue description from the report:
>>
>> When a page fault occurs in a secret memory file created with
>> `memfd_secret(2)`, the kernel will allocate a new folio for it, mark
>> the underlying page as not-present in the direct map, and add it to
>> the file mapping.
>>
>> If two tasks cause a fault in the same page concurrently, both could
>> end up allocating a folio and removing the page from the direct map,
>> but only one would succeed in adding the folio to the file
>> mapping. The task that failed undoes the effects of its attempt by (a)
>> freeing the folio again and (b) putting the page back into the direct
>> map. However, by doing these two operations in this order, the page
>> becomes available to the allocator again before it is placed back in
>> the direct mapping.
>>
>> If another task attempts to allocate the page between (a) and (b), and
>> the kernel tries to access it via the direct map, it would result in a
>> supervisor not-present page fault.
>>> Fix the ordering to restore the map before the folio is freed.
>>
>> ... restore the direct map
>>
>> With these changes
>>
>> Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
> 
> Fully agreed
> 
> Acked-by: David Hildenbrand <david@redhat.com>

Cheers!

Re: [PATCH 1/1] mm/secretmem: fix use-after-free race in fault handler

[Lorenzo Stoakes]

Small thing, sorry to be a pain buuuut could we please not send patches
in-reply to another mail, it makes it harder for people to see :)

On Fri, Oct 31, 2025 at 11:59:16AM +0200, Mike Rapoport wrote:
> On Fri, Oct 31, 2025 at 05:18:18PM +0800, Lance Yang wrote:
> > From: Lance Yang <lance.yang@linux.dev>
> >
> > The error path in secretmem_fault() frees a folio before restoring its
> > direct map status, which is a race leading to a panic.
>
> Let's use the issue description from the report:
>
> When a page fault occurs in a secret memory file created with
> `memfd_secret(2)`, the kernel will allocate a new folio for it, mark
> the underlying page as not-present in the direct map, and add it to
> the file mapping.
>
> If two tasks cause a fault in the same page concurrently, both could
> end up allocating a folio and removing the page from the direct map,
> but only one would succeed in adding the folio to the file
> mapping. The task that failed undoes the effects of its attempt by (a)
> freeing the folio again and (b) putting the page back into the direct
> map. However, by doing these two operations in this order, the page
> becomes available to the allocator again before it is placed back in
> the direct mapping.
>
> If another task attempts to allocate the page between (a) and (b), and
> the kernel tries to access it via the direct map, it would result in a
> supervisor not-present page fault.
>
> > Fix the ordering to restore the map before the folio is freed.
>
> ... restore the direct map
>
> With these changes
>
> Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

Agree with David, Mike this looks 'obviously correct' thanks for addressing
it.

But also as per Mike, please update message accordingly and send v2
not-in-reply-to-anything :P

With that said:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

>
> >
> > Cc: <stable@vger.kernel.org>
> > Reported-by: Google Big Sleep <big-sleep-vuln-reports@google.com>
> > Closes: https://lore.kernel.org/linux-mm/CAEXGt5QeDpiHTu3K9tvjUTPqo+d-=wuCNYPa+6sWKrdQJ-ATdg@mail.gmail.com/
> > Signed-off-by: Lance Yang <lance.yang@linux.dev>
> > ---
> >  mm/secretmem.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/mm/secretmem.c b/mm/secretmem.c
> > index c1bd9a4b663d..37f6d1097853 100644
> > --- a/mm/secretmem.c
> > +++ b/mm/secretmem.c
> > @@ -82,13 +82,13 @@ static vm_fault_t secretmem_fault(struct vm_fault *vmf)
> >  		__folio_mark_uptodate(folio);
> >  		err = filemap_add_folio(mapping, folio, offset, gfp);
> >  		if (unlikely(err)) {
> > -			folio_put(folio);
> >  			/*
> >  			 * If a split of large page was required, it
> >  			 * already happened when we marked the page invalid
> >  			 * which guarantees that this call won't fail
> >  			 */
> >  			set_direct_map_default_noflush(folio_page(folio, 0));
> > +			folio_put(folio);
> >  			if (err == -EEXIST)
> >  				goto retry;
> >
> > --
> > 2.49.0
> >
>
> --
> Sincerely yours,
> Mike.

Re: [PATCH 1/1] mm/secretmem: fix use-after-free race in fault handler

[Lance Yang]

On 2025/10/31 18:24, Lorenzo Stoakes wrote:
> Small thing, sorry to be a pain buuuut could we please not send patches
> in-reply to another mail, it makes it harder for people to see :)
> 
> On Fri, Oct 31, 2025 at 11:59:16AM +0200, Mike Rapoport wrote:
>> On Fri, Oct 31, 2025 at 05:18:18PM +0800, Lance Yang wrote:
>>> From: Lance Yang <lance.yang@linux.dev>
>>>
>>> The error path in secretmem_fault() frees a folio before restoring its
>>> direct map status, which is a race leading to a panic.
>>
>> Let's use the issue description from the report:
>>
>> When a page fault occurs in a secret memory file created with
>> `memfd_secret(2)`, the kernel will allocate a new folio for it, mark
>> the underlying page as not-present in the direct map, and add it to
>> the file mapping.
>>
>> If two tasks cause a fault in the same page concurrently, both could
>> end up allocating a folio and removing the page from the direct map,
>> but only one would succeed in adding the folio to the file
>> mapping. The task that failed undoes the effects of its attempt by (a)
>> freeing the folio again and (b) putting the page back into the direct
>> map. However, by doing these two operations in this order, the page
>> becomes available to the allocator again before it is placed back in
>> the direct mapping.
>>
>> If another task attempts to allocate the page between (a) and (b), and
>> the kernel tries to access it via the direct map, it would result in a
>> supervisor not-present page fault.
>>
>>> Fix the ordering to restore the map before the folio is freed.
>>
>> ... restore the direct map
>>
>> With these changes
>>
>> Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
> 
> Agree with David, Mike this looks 'obviously correct' thanks for addressing
> it.
> 
> But also as per Mike, please update message accordingly and send v2
> not-in-reply-to-anything :P

Sure. V2 is on the way ;)

> 
> With that said:
> 
> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

Thanks!
Lance

> 
>>
>>>
>>> Cc: <stable@vger.kernel.org>
>>> Reported-by: Google Big Sleep <big-sleep-vuln-reports@google.com>
>>> Closes: https://lore.kernel.org/linux-mm/CAEXGt5QeDpiHTu3K9tvjUTPqo+d-=wuCNYPa+6sWKrdQJ-ATdg@mail.gmail.com/
>>> Signed-off-by: Lance Yang <lance.yang@linux.dev>
>>> ---
>>>   mm/secretmem.c | 2 +-
>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/mm/secretmem.c b/mm/secretmem.c
>>> index c1bd9a4b663d..37f6d1097853 100644
>>> --- a/mm/secretmem.c
>>> +++ b/mm/secretmem.c
>>> @@ -82,13 +82,13 @@ static vm_fault_t secretmem_fault(struct vm_fault *vmf)
>>>   		__folio_mark_uptodate(folio);
>>>   		err = filemap_add_folio(mapping, folio, offset, gfp);
>>>   		if (unlikely(err)) {
>>> -			folio_put(folio);
>>>   			/*
>>>   			 * If a split of large page was required, it
>>>   			 * already happened when we marked the page invalid
>>>   			 * which guarantees that this call won't fail
>>>   			 */
>>>   			set_direct_map_default_noflush(folio_page(folio, 0));
>>> +			folio_put(folio);
>>>   			if (err == -EEXIST)
>>>   				goto retry;
>>>
>>> --
>>> 2.49.0
>>>
>>
>> --
>> Sincerely yours,
>> Mike.

Re: [PATCH 1/1] mm/secretmem: fix use-after-free race in fault handler

[Lance Yang]

On 2025/10/31 17:59, Mike Rapoport wrote:
> On Fri, Oct 31, 2025 at 05:18:18PM +0800, Lance Yang wrote:
>> From: Lance Yang <lance.yang@linux.dev>
>>
>> The error path in secretmem_fault() frees a folio before restoring its
>> direct map status, which is a race leading to a panic.
> 
> Let's use the issue description from the report:

Will do. I'll also add the missing Fixes: tag.

> 
> When a page fault occurs in a secret memory file created with
> `memfd_secret(2)`, the kernel will allocate a new folio for it, mark
> the underlying page as not-present in the direct map, and add it to
> the file mapping.
> 
> If two tasks cause a fault in the same page concurrently, both could
> end up allocating a folio and removing the page from the direct map,
> but only one would succeed in adding the folio to the file
> mapping. The task that failed undoes the effects of its attempt by (a)
> freeing the folio again and (b) putting the page back into the direct
> map. However, by doing these two operations in this order, the page
> becomes available to the allocator again before it is placed back in
> the direct mapping.
> 
> If another task attempts to allocate the page between (a) and (b), and
> the kernel tries to access it via the direct map, it would result in a
> supervisor not-present page fault.
>   
>> Fix the ordering to restore the map before the folio is freed.
> 
> ... restore the direct map
> 
> With these changes
> 
> Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

Thanks!
Lance

> 
>>
>> Cc: <stable@vger.kernel.org>
>> Reported-by: Google Big Sleep <big-sleep-vuln-reports@google.com>
>> Closes: https://lore.kernel.org/linux-mm/CAEXGt5QeDpiHTu3K9tvjUTPqo+d-=wuCNYPa+6sWKrdQJ-ATdg@mail.gmail.com/
>> Signed-off-by: Lance Yang <lance.yang@linux.dev>
>> ---
>>   mm/secretmem.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/mm/secretmem.c b/mm/secretmem.c
>> index c1bd9a4b663d..37f6d1097853 100644
>> --- a/mm/secretmem.c
>> +++ b/mm/secretmem.c
>> @@ -82,13 +82,13 @@ static vm_fault_t secretmem_fault(struct vm_fault *vmf)
>>   		__folio_mark_uptodate(folio);
>>   		err = filemap_add_folio(mapping, folio, offset, gfp);
>>   		if (unlikely(err)) {
>> -			folio_put(folio);
>>   			/*
>>   			 * If a split of large page was required, it
>>   			 * already happened when we marked the page invalid
>>   			 * which guarantees that this call won't fail
>>   			 */
>>   			set_direct_map_default_noflush(folio_page(folio, 0));
>> +			folio_put(folio);
>>   			if (err == -EEXIST)
>>   				goto retry;
>>   
>> -- 
>> 2.49.0
>>
>