Re: [PATCH] mm/hugetlb: Don't crash when allocating a folio if there are no resv

[Oscar Salvador <osalvador@suse.de>]

On Tue, Jun 17, 2025 at 10:28:40PM -0700, Vivek Kasireddy wrote:
> There are cases when we try to pin a folio but discover that it has
> not been faulted-in. So, we try to allocate it in memfd_alloc_folio()
> but there is a chance that we might encounter a fatal crash/failure
> (VM_BUG_ON(!h->resv_huge_pages) in alloc_hugetlb_folio_reserve()) if
> there are no active reservations at that instant. This issue was
> reported by syzbot:
> 
> kernel BUG at mm/hugetlb.c:2403!
> Oops: invalid opcode: 0000 [#1] PREEMPT SMP KASAN NOPTI
> CPU: 0 UID: 0 PID: 5315 Comm: syz.0.0 Not tainted
> 6.13.0-rc5-syzkaller-00161-g63676eefb7a0 #0
> Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS
> 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014
> RIP: 0010:alloc_hugetlb_folio_reserve+0xbc/0xc0 mm/hugetlb.c:2403
> Code: 1f eb 05 e8 56 18 a0 ff 48 c7 c7 40 56 61 8e e8 ba 21 cc 09 4c 89
> f0 5b 41 5c 41 5e 41 5f 5d c3 cc cc cc cc e8 35 18 a0 ff 90 <0f> 0b 66
> 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 f3 0f
> RSP: 0018:ffffc9000d3d77f8 EFLAGS: 00010087
> RAX: ffffffff81ff6beb RBX: 0000000000000000 RCX: 0000000000100000
> RDX: ffffc9000e51a000 RSI: 00000000000003ec RDI: 00000000000003ed
> RBP: 1ffffffff34810d9 R08: ffffffff81ff6ba3 R09: 1ffffd4000093005
> R10: dffffc0000000000 R11: fffff94000093006 R12: dffffc0000000000
> R13: dffffc0000000000 R14: ffffea0000498000 R15: ffffffff9a4086c8
> FS:  00007f77ac12e6c0(0000) GS:ffff88801fc00000(0000)
> knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007f77ab54b170 CR3: 0000000040b70000 CR4: 0000000000352ef0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Call Trace:
>  <TASK>
>  memfd_alloc_folio+0x1bd/0x370 mm/memfd.c:88
>  memfd_pin_folios+0xf10/0x1570 mm/gup.c:3750
>  udmabuf_pin_folios drivers/dma-buf/udmabuf.c:346 [inline]
>  udmabuf_create+0x70e/0x10c0 drivers/dma-buf/udmabuf.c:443
>  udmabuf_ioctl_create drivers/dma-buf/udmabuf.c:495 [inline]
>  udmabuf_ioctl+0x301/0x4e0 drivers/dma-buf/udmabuf.c:526
>  vfs_ioctl fs/ioctl.c:51 [inline]
>  __do_sys_ioctl fs/ioctl.c:906 [inline]
>  __se_sys_ioctl+0xf5/0x170 fs/ioctl.c:892
>  do_syscall_x64 arch/x86/entry/common.c:52 [inline]
>  do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> 
> Therefore, prevent the above crash by replacing the VM_BUG_ON()
> with WARN_ON_ONCE() as there is no need to crash the system in
> this situation and instead we could just warn and fail the
> allocation.
> 
> Fixes: 26a8ea80929c ("mm/hugetlb: fix memfd_pin_folios resv_huge_pages leak")
> Reported-by: syzbot+a504cb5bae4fe117ba94@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=a504cb5bae4fe117ba94
> Cc: Steve Sistare <steven.sistare@oracle.com>
> Cc: Muchun Song <muchun.song@linux.dev>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Signed-off-by: Vivek Kasireddy <vivek.kasireddy@intel.com>

Who is supossed to reserve these hugepages?
hugetlb_reserve_pages() is only called at mmap/file setup, and you mention that
you try to allocate the folios even before mmap, so who's in charge of
making those reservations for you?

 

-- 
Oscar Salvador
SUSE Labs