[PATCH] memfd: deny writeable mappings when implying SEAL_WRITE

[PATCH] memfd: deny writeable mappings when implying SEAL_WRITE

[Pratyush Yadav]

From: "Pratyush Yadav (Google)" <pratyush@kernel.org>

When SEAL_EXEC is added, SEAL_WRITE is implied to make W^X. But the
implied seal is set after the check that makes sure the memfd can not
have any writable mappings. This means one can use SEAL_EXEC to apply
SEAL_WRITE while having writeable mappings.

This breaks the contract that SEAL_WRITE provides and can be used by an
attacker to pass a memfd that appears to be write sealed but can still
be modified arbitrarily.

Fix this by adding the implied seals before the call for
mapping_deny_writable() is done.

Fixes: c4f75bc8bd6b ("mm/memfd: add write seals when apply SEAL_EXEC to executable memfd")
Cc: stable@vger.kernel.org
Signed-off-by: Pratyush Yadav (Google) <pratyush@kernel.org>
---
 mm/memfd.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/mm/memfd.c b/mm/memfd.c
index fb425f4e315f..abe13b291ddc 100644
--- a/mm/memfd.c
+++ b/mm/memfd.c
@@ -283,6 +283,12 @@ int memfd_add_seals(struct file *file, unsigned int seals)
 		goto unlock;
 	}
 
+	/*
+	 * SEAL_EXEC implies SEAL_WRITE, making W^X from the start.
+	 */
+	if (seals & F_SEAL_EXEC && inode->i_mode & 0111)
+		seals |= F_SEAL_SHRINK|F_SEAL_GROW|F_SEAL_WRITE|F_SEAL_FUTURE_WRITE;
+
 	if ((seals & F_SEAL_WRITE) && !(*file_seals & F_SEAL_WRITE)) {
 		error = mapping_deny_writable(file->f_mapping);
 		if (error)
@@ -295,12 +301,6 @@ int memfd_add_seals(struct file *file, unsigned int seals)
 		}
 	}
 
-	/*
-	 * SEAL_EXEC implies SEAL_WRITE, making W^X from the start.
-	 */
-	if (seals & F_SEAL_EXEC && inode->i_mode & 0111)
-		seals |= F_SEAL_SHRINK|F_SEAL_GROW|F_SEAL_WRITE|F_SEAL_FUTURE_WRITE;
-
 	*file_seals |= seals;
 	error = 0;
 
-- 
2.54.0.545.g6539524ca2-goog

Re: [PATCH] memfd: deny writeable mappings when implying SEAL_WRITE

[Pasha Tatashin]

On 05-05 15:39, Pratyush Yadav wrote:
> From: "Pratyush Yadav (Google)" <pratyush@kernel.org>
> 
> When SEAL_EXEC is added, SEAL_WRITE is implied to make W^X. But the
> implied seal is set after the check that makes sure the memfd can not
> have any writable mappings. This means one can use SEAL_EXEC to apply
> SEAL_WRITE while having writeable mappings.
> 
> This breaks the contract that SEAL_WRITE provides and can be used by an
> attacker to pass a memfd that appears to be write sealed but can still
> be modified arbitrarily.
> 
> Fix this by adding the implied seals before the call for
> mapping_deny_writable() is done.
> 
> Fixes: c4f75bc8bd6b ("mm/memfd: add write seals when apply SEAL_EXEC to executable memfd")
> Cc: stable@vger.kernel.org
> Signed-off-by: Pratyush Yadav (Google) <pratyush@kernel.org>

Reviewed-by: Pasha Tatashin <pasha.tatashin@soleen.com> 

> ---
>  mm/memfd.c | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/memfd.c b/mm/memfd.c
> index fb425f4e315f..abe13b291ddc 100644
> --- a/mm/memfd.c
> +++ b/mm/memfd.c
> @@ -283,6 +283,12 @@ int memfd_add_seals(struct file *file, unsigned int seals)
>  		goto unlock;
>  	}
>  
> +	/*
> +	 * SEAL_EXEC implies SEAL_WRITE, making W^X from the start.
> +	 */
> +	if (seals & F_SEAL_EXEC && inode->i_mode & 0111)
> +		seals |= F_SEAL_SHRINK|F_SEAL_GROW|F_SEAL_WRITE|F_SEAL_FUTURE_WRITE;
> +
>  	if ((seals & F_SEAL_WRITE) && !(*file_seals & F_SEAL_WRITE)) {
>  		error = mapping_deny_writable(file->f_mapping);
>  		if (error)
> @@ -295,12 +301,6 @@ int memfd_add_seals(struct file *file, unsigned int seals)
>  		}
>  	}
>  
> -	/*
> -	 * SEAL_EXEC implies SEAL_WRITE, making W^X from the start.
> -	 */
> -	if (seals & F_SEAL_EXEC && inode->i_mode & 0111)
> -		seals |= F_SEAL_SHRINK|F_SEAL_GROW|F_SEAL_WRITE|F_SEAL_FUTURE_WRITE;
> -
>  	*file_seals |= seals;
>  	error = 0;
>  
> -- 
> 2.54.0.545.g6539524ca2-goog
> 

Re: [PATCH] memfd: deny writeable mappings when implying SEAL_WRITE

[Pasha Tatashin]

On Tue, 05 May 2026 15:39:20 +0200, Pratyush Yadav wrote:
> When SEAL_EXEC is added, SEAL_WRITE is implied to make W^X. But the
> implied seal is set after the check that makes sure the memfd can not
> have any writable mappings. This means one can use SEAL_EXEC to apply
> SEAL_WRITE while having writeable mappings.
> 
> This breaks the contract that SEAL_WRITE provides and can be used by an
> attacker to pass a memfd that appears to be write sealed but can still
> be modified arbitrarily.
> 
> [...]

Applied, thanks!

[1/1] memfd: deny writeable mappings when implying SEAL_WRITE
      commit: 73f496662a9848021e75742a69a3239ea850c3ee

Best regards,
-- 
Pasha Tatashin <pasha.tatashin@soleen.com>

Re: [PATCH] memfd: deny writeable mappings when implying SEAL_WRITE

[Pasha Tatashin]

On 05-05 15:37, Pasha Tatashin wrote:
> 
> On Tue, 05 May 2026 15:39:20 +0200, Pratyush Yadav wrote:
> > When SEAL_EXEC is added, SEAL_WRITE is implied to make W^X. But the
> > implied seal is set after the check that makes sure the memfd can not
> > have any writable mappings. This means one can use SEAL_EXEC to apply
> > SEAL_WRITE while having writeable mappings.
> > 
> > This breaks the contract that SEAL_WRITE provides and can be used by an
> > attacker to pass a memfd that appears to be write sealed but can still
> > be modified arbitrarily.
> > 
> > [...]
> 
> Applied, thanks!
> 
> [1/1] memfd: deny writeable mappings when implying SEAL_WRITE
>       commit: 73f496662a9848021e75742a69a3239ea850c3ee

^^^
Please ignore, this should be Applied to MM tree.

Pasha

> 
> Best regards,
> -- 
> Pasha Tatashin <pasha.tatashin@soleen.com>
> 

Re: [PATCH] memfd: deny writeable mappings when implying SEAL_WRITE

[Jeff Xu]

Hi Pratyush,

Thank you for fixing this.

On Tue, May 5, 2026 at 6:39 AM Pratyush Yadav <pratyush@kernel.org> wrote:
>
> From: "Pratyush Yadav (Google)" <pratyush@kernel.org>
>
> When SEAL_EXEC is added, SEAL_WRITE is implied to make W^X. But the
> implied seal is set after the check that makes sure the memfd can not
> have any writable mappings. This means one can use SEAL_EXEC to apply
> SEAL_WRITE while having writeable mappings.
>
> This breaks the contract that SEAL_WRITE provides and can be used by an
> attacker to pass a memfd that appears to be write sealed but can still
> be modified arbitrarily.
>
> Fix this by adding the implied seals before the call for
> mapping_deny_writable() is done.
>
> Fixes: c4f75bc8bd6b ("mm/memfd: add write seals when apply SEAL_EXEC to executable memfd")
> Cc: stable@vger.kernel.org
> Signed-off-by: Pratyush Yadav (Google) <pratyush@kernel.org>
Acked-by: Jeff Xu <jeffxu@google.com>

-Jeff

> ---
>  mm/memfd.c | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/mm/memfd.c b/mm/memfd.c
> index fb425f4e315f..abe13b291ddc 100644
> --- a/mm/memfd.c
> +++ b/mm/memfd.c
> @@ -283,6 +283,12 @@ int memfd_add_seals(struct file *file, unsigned int seals)
>                 goto unlock;
>         }
>
> +       /*
> +        * SEAL_EXEC implies SEAL_WRITE, making W^X from the start.
> +        */
> +       if (seals & F_SEAL_EXEC && inode->i_mode & 0111)
> +               seals |= F_SEAL_SHRINK|F_SEAL_GROW|F_SEAL_WRITE|F_SEAL_FUTURE_WRITE;
> +
>         if ((seals & F_SEAL_WRITE) && !(*file_seals & F_SEAL_WRITE)) {
>                 error = mapping_deny_writable(file->f_mapping);
>                 if (error)
> @@ -295,12 +301,6 @@ int memfd_add_seals(struct file *file, unsigned int seals)
>                 }
>         }
>
> -       /*
> -        * SEAL_EXEC implies SEAL_WRITE, making W^X from the start.
> -        */
> -       if (seals & F_SEAL_EXEC && inode->i_mode & 0111)
> -               seals |= F_SEAL_SHRINK|F_SEAL_GROW|F_SEAL_WRITE|F_SEAL_FUTURE_WRITE;
> -
>         *file_seals |= seals;
>         error = 0;
>
> --
> 2.54.0.545.g6539524ca2-goog
>