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memfd: deny writeable mappings when implying SEAL_WRITE

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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.

Link: https://lore.kernel.org/20260505133922.797635-1-pratyush@kernel.org
Fixes: c4f75bc8bd ("mm/memfd: add write seals when apply SEAL_EXEC to executable memfd")
Signed-off-by: Pratyush Yadav (Google) <pratyush@kernel.org>
Reviewed-by: Pasha Tatashin <pasha.tatashin@soleen.com> 
Acked-by: Jeff Xu <jeffxu@google.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Brendan Jackman <jackmanb@google.com>
Cc: Greg Thelen <gthelen@google.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Kees Cook <kees@kernel.org>
Cc: "David Hildenbrand (Arm)" <david@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Pratyush Yadav (Google) 2026-05-05 15:39:20 +02:00 committed by Andrew Morton
parent fa0b9b2b7a
commit 3b041514cb
1 changed files with 6 additions and 6 deletions
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12
mm/memfd.c
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@ -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;