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futex: Fix UaF between futex_key_to_node_opt() and vma_replace_policy()

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During futex_key_to_node_opt() execution, vma->vm_policy is read under
speculative mmap lock and RCU. Concurrently, mbind() may call
vma_replace_policy() which frees the old mempolicy immediately via
kmem_cache_free().

This creates a race where __futex_key_to_node() dereferences a freed
mempolicy pointer, causing a use-after-free read of mpol->mode.

[  151.412631] BUG: KASAN: slab-use-after-free in __futex_key_to_node (kernel/futex/core.c:349)
[  151.414046] Read of size 2 at addr ffff888001c49634 by task e/87

[  151.415969] Call Trace:

[  151.416732]  __asan_load2 (mm/kasan/generic.c:271)
[  151.416777]  __futex_key_to_node (kernel/futex/core.c:349)
[  151.416822]  get_futex_key (kernel/futex/core.c:374 kernel/futex/core.c:386 kernel/futex/core.c:593)

Fix by adding rcu to __mpol_put().

Fixes: c042c50521 ("futex: Implement FUTEX2_MPOL")
Reported-by: Hao-Yu Yang <naup96721@gmail.com>
Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Hao-Yu Yang <naup96721@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Link: https://patch.msgid.link/20260324174418.GB1850007@noisy.programming.kicks-ass.net
This commit is contained in:
Hao-Yu Yang
2026-03-13 20:47:56 +08:00
committed by Peter Zijlstra
parent 19f94b3905
commit 190a8c48ff
3 changed files with 10 additions and 3 deletions
Download Patch File Download Diff File Expand all files Collapse all files

1
include/linux/mempolicy.h
View File

@@ -55,6 +55,7 @@ struct mempolicy {
nodemask_t cpuset_mems_allowed; /* relative to these nodes */
nodemask_t user_nodemask; /* nodemask passed by user */
} w;
struct rcu_head rcu;
};
/*

2
kernel/futex/core.c
View File

@@ -342,7 +342,7 @@ static int __futex_key_to_node(struct mm_struct *mm, unsigned long addr)
if (!vma)
return FUTEX_NO_NODE;
mpol = vma_policy(vma);
mpol = READ_ONCE(vma->vm_policy);
if (!mpol)
return FUTEX_NO_NODE;

10
mm/mempolicy.c
View File

@@ -487,7 +487,13 @@ void __mpol_put(struct mempolicy *pol)
{
if (!atomic_dec_and_test(&pol->refcnt))
return;
kmem_cache_free(policy_cache, pol);
/*
* Required to allow mmap_lock_speculative*() access, see for example
* futex_key_to_node_opt(). All accesses are serialized by mmap_lock,
* however the speculative lock section unbound by the normal lock
* boundaries, requiring RCU freeing.
*/
kfree_rcu(pol, rcu);
}
EXPORT_SYMBOL_FOR_MODULES(__mpol_put, "kvm");
@@ -1020,7 +1026,7 @@ static int vma_replace_policy(struct vm_area_struct *vma,
}
old = vma->vm_policy;
vma->vm_policy = new; /* protected by mmap_lock */
WRITE_ONCE(vma->vm_policy, new); /* protected by mmap_lock */
mpol_put(old);
return 0;