linux-kernel - Re: [PATCH v3 1/2] mm/ksm: fix flag-dropping behavior in ksm

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Message-ID: <0bc6a1ba-4f4f-4b04-b66c-b5d217faefab@kernel.org>
Date: Thu, 6 Nov 2025 12:16:36 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: Vlastimil Babka <vbabka@...e.cz>, Jakub Acs <acsjakub@...zon.de>,
 linux-mm@...ck.org, Hugh Dickins <hughd@...gle.com>,
 Jann Horn <jannh@...gle.com>, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: akpm@...ux-foundation.org, xu.xin16@....com.cn, chengming.zhou@...ux.dev,
 peterx@...hat.com, axelrasmussen@...gle.com, linux-kernel@...r.kernel.org,
 stable@...r.kernel.org
Subject: Re: [PATCH v3 1/2] mm/ksm: fix flag-dropping behavior in ksm_madvise

On 06.11.25 11:39, Vlastimil Babka wrote:
> On 10/1/25 11:03, 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 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.
>>
>> Fixes: 7677f7fd8be76 ("userfaultfd: add minor fault registration mode")
> 
> Late to the party, but it seems to me the correct Fixes: should be
> f8af4da3b4c1 ("ksm: the mm interface to ksm")
> which introduced the flag and the buggy clearing code, no?
> 
> Commit 7677f7fd8be76 is just one that notices it, right? But there are other
> flags in >32 bit area, including pkeys etc. Sounds rather dangerous if they
> can be cleared using a madvise.
> 
> So we can't amend the Fixes: now but maybe could advise stable to backport
> for even older versions than based on 7677f7fd8be76 ?

Yes, I agree.

-- 
Cheers

David