Re: [PATCH] mm/ksm: fix flag-dropping behavior in ksm_madvise

[David Hildenbrand <david@redhat.com>]

On 30.09.25 08:39, Jakub Acs wrote:
> 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 casting `VM_MERGEABLE` constant to unsigned long to preserve
> the upper 32 bits, in case it's needed.
> 
> 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.
> 
> Fixes: 7677f7fd8be76 ("userfaultfd: add minor fault registration mode")
> Signed-off-by: Jakub Acs <acsjakub@amazon.de>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: David Hildenbrand <david@redhat.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: Mike Kravetz <mike.kravetz@oracle.com>
> Cc: linux-mm@kvack.org
> Cc: linux-kernel@vger.kernel.org
> Cc: stable@vger.kernel.org
> ---
> 
> I looked around the kernel and found one more flag that might be
> causing similar issues: "IORESOURCE_BUSY" - as its inverted version is
> bit-anded to unsigned long fields. However, it seems those fields don't
> actually use any bits from upper 32-bits as flags (yet?).
> 
> I also considered changing the constant definition by adding ULL, but am
> not sure where else that could blow up, plus it would likely call to
> define all the related constants as ULL for consistency. If you'd prefer
> that fix, let me know.
> 
> 
>   mm/ksm.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/ksm.c b/mm/ksm.c
> index 160787bb121c..c24137a1eeb7 100644
> --- a/mm/ksm.c
> +++ b/mm/ksm.c
> @@ -2871,7 +2871,7 @@ int ksm_madvise(struct vm_area_struct *vma, unsigned long start,
>   				return err;
>   		}
>   
> -		*vm_flags &= ~VM_MERGEABLE;
> +		*vm_flags &= ~((unsigned long) VM_MERGEABLE);
>   		break;
>   	}
>   

Wouldn't it be better to just do

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 1ae97a0b8ec75..0eaf8af153f98 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -296,7 +296,7 @@ extern unsigned int kobjsize(const void *objp);
  #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   0x80000000ul    /* 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 */


And for consistency doing it to all other flags as well? After all we have

	typedef unsigned long vm_flags_t;

-- 
Cheers

David / dhildenb