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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: > > > > 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=3Da504cb5bae4fe117ba94 > > Cc: Steve Sistare > > Cc: Muchun Song > > Cc: David Hildenbrand > > Cc: Andrew Morton > > Signed-off-by: Vivek Kasireddy >=20 > Who is supossed to reserve these hugepages? > hugetlb_reserve_pages() is only called at mmap/file setup, and you mentio= n > that > you try to allocate the folios even before mmap, so who's in charge of > making those reservations for you? In this specific case, I would say the caller (memfd_alloc_folio()) should = be the one making the reservation before it tries to allocate the folio. And, the = other series you commented on is trying to do just that. However, as mentioned in the other thread (replying to Andrew and Anshuman)= , this is a very uncommon use-case as hugetlbfs_file_mmap() is not called fir= st. So, this patch is only trying to prevent the crash but the actual underlyin= g problem (no reservation) is addressed in the other series. Thanks, Vivek >=20 >=20 >=20 > -- > Oscar Salvador > SUSE Labs