[PATCH v1] futex: Use-after-free between futex_key_to_node_opt and vma_replace_policy

[Hao-Yu Yang]

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
[  151.414476]
[  151.415431] CPU: 1 UID: 1000 PID: 87 Comm: e Not tainted 7.0.0-rc3-g0257f64bdac7 #1 PREEMPT(lazy)
[  151.415758] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1 04/01/2014
[  151.415969] Call Trace:
[  151.416059]  <TASK>
[  151.416161]  dump_stack_lvl (lib/dump_stack.c:123)
[  151.416299]  print_report (mm/kasan/report.c:379 mm/kasan/report.c:482)
[  151.416359]  ? __virt_addr_valid (./include/linux/mmzone.h:2046 ./include/linux/mmzone.h:2198 arch/x86/mm/physaddr.c:54)
[  151.416412]  ? __futex_key_to_node (kernel/futex/core.c:349)
[  151.416517]  ? kasan_complete_mode_report_info (mm/kasan/report_generic.c:182)
[  151.416583]  ? __futex_key_to_node (kernel/futex/core.c:349)
[  151.416631]  kasan_report (mm/kasan/report.c:597)
[  151.416677]  ? __futex_key_to_node (kernel/futex/core.c:349)
[  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)
[  151.416871]  ? __pfx_get_futex_key (kernel/futex/core.c:550)
[  151.416927]  futex_wake (kernel/futex/waitwake.c:165)
[  151.416976]  ? __pfx_futex_wake (kernel/futex/waitwake.c:156)
[  151.417022]  ? __pfx___x64_sys_futex_wait (kernel/futex/syscalls.c:398)
[  151.417081]  __x64_sys_futex_wake (kernel/futex/syscalls.c:382 kernel/futex/syscalls.c:366 kernel/futex/syscalls.c:366)
[  151.417129]  x64_sys_call (arch/x86/entry/syscall_64.c:41)
[  151.417236]  do_syscall_64 (arch/x86/entry/syscall_64.c:63 arch/x86/entry/syscall_64.c:94)
[  151.417342]  entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:130)
[  151.418312]  </TASK>

I don't really know how to fix this vulnerability. So this
patch just initial attempt to patch and need more discussion.
I think we need to remove this path or add some lock.

Fixes: c042c505210d ("futex: Implement FUTEX2_MPOL")
Reported-by: Hao-Yu Yang <naup96721@xxxxxxxxx>
Signed-off-by: Hao-Yu Yang <naup96721@xxxxxxxxx>
---
 kernel/futex/core.c | 23 -----------------------
 1 file changed, 23 deletions(-)

diff --git a/kernel/futex/core.c b/kernel/futex/core.c
index cf7e610eac42..0b44fcb30268 100644
--- a/kernel/futex/core.c
+++ b/kernel/futex/core.c
@@ -362,31 +362,8 @@ static int __futex_key_to_node(struct mm_struct *mm, unsigned long addr)
 	return node;
 }
 
-static int futex_key_to_node_opt(struct mm_struct *mm, unsigned long addr)
-{
-	int seq, node;
-
-	guard(rcu)();
-
-	if (!mmap_lock_speculate_try_begin(mm, &seq))
-		return -EBUSY;
-
-	node = __futex_key_to_node(mm, addr);
-
-	if (mmap_lock_speculate_retry(mm, seq))
-		return -EAGAIN;
-
-	return node;
-}
-
 static int futex_mpol(struct mm_struct *mm, unsigned long addr)
 {
-	int node;
-
-	node = futex_key_to_node_opt(mm, addr);
-	if (node >= FUTEX_NO_NODE)
-		return node;
-
 	guard(mmap_read_lock)(mm);
 	return __futex_key_to_node(mm, addr);
 }
-- 
2.34.1