Re: [PATCH] mm/memfd: reserve hugetlb folios before allocation

[Steven Sistare <steven.sistare@oracle.com>]

On 1/7/2025 2:25 AM, 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 crash/failure
> (VM_BUG_ON(!h->resv_huge_pages)) 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, to avoid this situation and fix this issue, we just need
> to make a reservation before we try to allocate the folio. While at
> it, also remove the VM_BUG_ON() as there is no need to crash the
> system in this scenario and instead we could just fail the allocation.
> 
> Fixes: 26a8ea80929c ("mm/hugetlb: fix memfd_pin_folios resv_huge_pages leak")
> Reported-by: syzbot+a504cb5bae4fe117ba94@syzkaller.appspotmail.com
> Signed-off-by: Vivek Kasireddy <vivek.kasireddy@intel.com>
> 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>
> ---
>   mm/hugetlb.c | 9 ++++++---
>   mm/memfd.c   | 5 +++++
>   2 files changed, 11 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index c498874a7170..e46c461210a4 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -2397,12 +2397,15 @@ struct folio *alloc_hugetlb_folio_reserve(struct hstate *h, int preferred_nid,
>   	struct folio *folio;
>   
>   	spin_lock_irq(&hugetlb_lock);
> +	if (!h->resv_huge_pages) {
> +		spin_unlock_irq(&hugetlb_lock);
> +		return NULL;
> +	}

This should be the entire fix, plus deleting the VM_BUG_ON.  See below.

> +
>   	folio = dequeue_hugetlb_folio_nodemask(h, gfp_mask, preferred_nid,
>   					       nmask);
> -	if (folio) {
> -		VM_BUG_ON(!h->resv_huge_pages);
> +	if (folio)
>   		h->resv_huge_pages--;
> -	}
>   
>   	spin_unlock_irq(&hugetlb_lock);
>   	return folio;
> diff --git a/mm/memfd.c b/mm/memfd.c
> index 35a370d75c9a..a3012c444285 100644
> --- a/mm/memfd.c
> +++ b/mm/memfd.c
> @@ -85,6 +85,10 @@ struct folio *memfd_alloc_folio(struct file *memfd, pgoff_t idx)
>   		gfp_mask &= ~(__GFP_HIGHMEM | __GFP_MOVABLE);
>   		idx >>= huge_page_order(h);
>   
> +		if (!hugetlb_reserve_pages(file_inode(memfd),
> +					   idx, idx + 1, NULL, 0))
> +			return ERR_PTR(-ENOMEM);

I believe it is wrong to force a reservation here.  Pages should have already been
reserved at this point, eg by calls from hugetlbfs_file_mmap or hugetlb_file_setup.
syzcaller has forced its way down this path without calling those pre-requisites,
doing weird stuff as it should.

To fix, I suggest you simply fix alloc_hugetlb_folio_reserve as above.

- Steve

> +
>   		folio = alloc_hugetlb_folio_reserve(h,
>   						    numa_node_id(),
>   						    NULL,
> @@ -100,6 +104,7 @@ struct folio *memfd_alloc_folio(struct file *memfd, pgoff_t idx)
>   			folio_unlock(folio);
>   			return folio;
>   		}
> +		hugetlb_unreserve_pages(file_inode(memfd), idx, idx + 1, 1);
>   		return ERR_PTR(-ENOMEM);
>   	}
>   #endif