[PATCH 6.6 102/105] mm/ksm: fix flag-dropping behavior in ksm_madvise

Greg Kroah-Hartman
2025-10-21T19:57:57Z

From: Jakub Acs <acsjakub@amazon.de>

commit f04aad36a07cc17b7a5d5b9a2d386ce6fae63e93 upstream.

syzkaller discovered the following crash: (kernel BUG)

[ 44.607039] ------------[ cut here ]------------
[ 44.607422] kernel BUG at mm/userfaultfd.c:2067!
[ 44.608148] Oops: invalid opcode: 0000 [#1] SMP DEBUG_PAGEALLOC KASAN NOPTI
[ 44.608814] CPU: 1 UID: 0 PID: 2475 Comm: reproducer Not tainted 6.16.0-rc6 #1 PREEMPT(none)
[ 44.609635] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.3-0-ga6ed6b701f0a-prebuilt.qemu.org 04/01/2014
[ 44.610695] RIP: 0010:userfaultfd_release_all+0x3a8/0x460

<snip other registers, drop unreliable trace>

[ 44.617726] Call Trace:
[ 44.617926] <TASK>
[ 44.619284] userfaultfd_release+0xef/0x1b0
[ 44.620976] __fput+0x3f9/0xb60
[ 44.621240] fput_close_sync+0x110/0x210
[ 44.622222] __x64_sys_close+0x8f/0x120
[ 44.622530] do_syscall_64+0x5b/0x2f0
[ 44.622840] entry_SYSCALL_64_after_hwframe+0x76/0x7e
[ 44.623244] RIP: 0033:0x7f365bb3f227

Kernel panics because it detects UFFD inconsistency during
userfaultfd_release_all(). Specifically, a VMA which has a valid pointer
to vma->vm_userfaultfd_ctx, but no UFFD flags in vma->vm_flags.

The inconsistency is caused in ksm_madvise(): when user calls madvise()
with MADV_UNMEARGEABLE on a VMA that is registered for UFFD in MINOR mode,
it accidentally clears all flags stored in the upper 32 bits of
vma->vm_flags.

Assuming x86_64 kernel build, unsigned long is 64-bit and unsigned int and
int are 32-bit wide. This setup causes the following mishap during the &=
~VM_MERGEABLE assignment.

VM_MERGEABLE is a 32-bit constant of type unsigned int, 0x8000'0000.
After ~ is applied, it becomes 0x7fff'ffff unsigned int, which is then
promoted to unsigned long before the & operation. This promotion fills
upper 32 bits with leading 0s, as we're doing unsigned conversion (and
even for a signed conversion, this wouldn't help as the leading bit is 0).
& operation thus ends up AND-ing vm_flags with 0x0000'0000'7fff'ffff
instead of intended 0xffff'ffff'7fff'ffff and hence accidentally clears
the upper 32-bits of its value.

Fix it by changing `VM_MERGEABLE` constant to unsigned long, using the
BIT() macro.

Note: other VM_* flags are not affected: This only happens to the
VM_MERGEABLE flag, as the other VM_* flags are all constants of type int
and after ~ operation, they end up with leading 1 and are thus converted
to unsigned long with leading 1s.

Note 2:
After commit 31defc3b01d9 ("userfaultfd: remove (VM_)BUG_ON()s"), this is
no longer a kernel BUG, but a WARNING at the same place:

[ 45.595973] WARNING: CPU: 1 PID: 2474 at mm/userfaultfd.c:2067

but the root-cause (flag-drop) remains the same.

[akpm@linux-foundation.org: rust bindgen wasn't able to handle BIT(), from Miguel]
Link: https://lore.kernel.org/oe-kbuild-all/202510030449.VfSaAjvd-lkp@intel.com/
Link: https://lkml.kernel.org/r/20251001090353.57523-2-acsjakub@amazon.de
Fixes: 7677f7fd8be7 ("userfaultfd: add minor fault registration mode")
Signed-off-by: Jakub Acs <acsjakub@amazon.de>
Signed-off-by: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Acked-by: David Hildenbrand <david@redhat.com>
Acked-by: SeongJae Park <sj@kernel.org>
Tested-by: Alice Ryhl <aliceryhl@google.com>
Tested-by: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Cc: Xu Xin <xu.xin16@zte.com.cn>
Cc: Chengming Zhou <chengming.zhou@linux.dev>
Cc: Peter Xu <peterx@redhat.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
[acsjakub@amazon.de: adapt rust bindgen const to older versions]
Signed-off-by: Jakub Acs <acsjakub@amazon.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
include/linux/mm.h | 2 +-
rust/bindings/bindings_helper.h | 2 ++
rust/bindings/lib.rs | 1 +
3 files changed, 4 insertions(+), 1 deletion(-)

--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -315,7 +315,7 @@ extern unsigned int kobjsize(const void
#define VM_MIXEDMAP 0x10000000 /* Can contain "struct page" and pure PFN pages */
#define VM_HUGEPAGE 0x20000000 /* MADV_HUGEPAGE marked this vma */
#define VM_NOHUGEPAGE 0x40000000 /* MADV_NOHUGEPAGE marked this vma */
-#define VM_MERGEABLE 0x80000000 /* KSM may merge identical pages */
+#define VM_MERGEABLE BIT(31) /* KSM may merge identical pages */

#ifdef CONFIG_ARCH_USES_HIGH_VMA_FLAGS
#define VM_HIGH_ARCH_BIT_0 32 /* bit only usable on 64-bit architectures */
--- a/rust/bindings/bindings_helper.h
+++ b/rust/bindings/bindings_helper.h
@@ -12,8 +12,10 @@
#include <linux/refcount.h>
#include <linux/wait.h>
#include <linux/sched.h>
+#include <linux/mm.h>

/* `bindgen` gets confused at certain things. */
const size_t BINDINGS_ARCH_SLAB_MINALIGN = ARCH_SLAB_MINALIGN;
const gfp_t BINDINGS_GFP_KERNEL = GFP_KERNEL;
const gfp_t BINDINGS___GFP_ZERO = __GFP_ZERO;
+const vm_flags_t BINDINGS_VM_MERGEABLE = VM_MERGEABLE;
--- a/rust/bindings/lib.rs
+++ b/rust/bindings/lib.rs
@@ -51,3 +51,4 @@ pub use bindings_raw::*;

pub const GFP_KERNEL: gfp_t = BINDINGS_GFP_KERNEL;
pub const __GFP_ZERO: gfp_t = BINDINGS___GFP_ZERO;
+pub const VM_MERGEABLE: vm_flags_t = BINDINGS_VM_MERGEABLE;