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

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

On 1/8/2025 2:24 AM, Kasireddy, Vivek wrote:
> Hi Steve,
> 
>>> 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.
> Is there any particular reason why you believe a reservation here would be wrong?
> AFAICS, at the moment, we are not doing any region/subpool accounting before
> our folio allocation and this gets flagged in the form of elevated resv_huge_pages
> value (hugetlb_acct_memory() elevates it based on the return value of region_del())
> when hugetlb_unreserve_pages() eventually gets called.
> 
>> Pages should have already been
>> reserved at this point, eg by calls from hugetlbfs_file_mmap or hugetlb_file_setup.
> hugetlb_file_setup() does not reserve any pages as it passes in VM_NORESERVE.
> And, the case we are trying to address is exactly when hugetlbfs_file_mmap() does
> not get called before pinning. 

But you must not break the case where hugetlbfs_file_mmap was called first, which
reserves, then memfd_alloc_folio is called, which reserves again with your fix. Does
that work correctly, or do the counts go bad?

> So, when hugetlbfs_file_mmap() does eventually
> get called, I don't see any problem if it calls hugetlb_reserve_pages() again for the
> same range or overlapping ranges.

Does that work correctly, or do the counts go bad?

Please try those scenarios with your test program:  mmap + memfd_alloc_folio, and
memfd_alloc_folio + mmap.

- Steve

>> syzcaller has forced its way down this path without calling those pre-requisites,
>> doing weird stuff as it should.
> This issue is not very hard to reproduce. If we have free_huge_pages > 0 and
> resv_huge_pages = 0, and then we call memfd_pin_folios() before mmap()/
> hugetlbfs_file_mmap() we can easily encounter this issue. Furthermore, we
> should be able to allocate a folio in this scenario (as available_huge_pages > 0),
> which we would not be able to do if we don't call hugetlb_reserve_pages().
> Note that hugetlb_reserve_pages() actually elevates resv_huge_pages in
> this case and kind of gives a go-ahead for the allocation.
> 
> I have used a slightly modified udmabuf selftest to reproduce this issue which
> I'll send out as part of v2.
> 
> Thanks,
> Vivek
> 
>>
>> 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
>