Re: [PATCH v3 1/3] s390/pci: Fix s390_mmio_read/write syscall page fault handling
From: David Hildenbrand
Date: Tue Jun 11 2024 - 10:14:12 EST
On 11.06.24 15:23, Niklas Schnelle wrote:
On Tue, 2024-06-11 at 14:08 +0200, Niklas Schnelle wrote:
On Tue, 2024-06-11 at 13:21 +0200, Niklas Schnelle wrote:
On Wed, 2024-05-29 at 13:36 +0200, Niklas Schnelle wrote:
The s390 MMIO syscalls when using the classic PCI instructions do not
cause a page fault when follow_pte() fails due to the page not being
present. Besides being a general deficiency this breaks vfio-pci's mmap()
handling once VFIO_PCI_MMAP gets enabled as this lazily maps on first
access. Fix this by following a failed follow_pte() with
fixup_user_page() and retrying the follow_pte().
Reviewed-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
Reviewed-by: Matthew Rosato <mjrosato@xxxxxxxxxxxxx>
Signed-off-by: Niklas Schnelle <schnelle@xxxxxxxxxxxxx>
---
arch/s390/pci/pci_mmio.c | 18 +++++++++++++-----
1 file changed, 13 insertions(+), 5 deletions(-)
diff --git a/arch/s390/pci/pci_mmio.c b/arch/s390/pci/pci_mmio.c
index 5398729bfe1b..80c21b1a101c 100644
--- a/arch/s390/pci/pci_mmio.c
+++ b/arch/s390/pci/pci_mmio.c
@@ -170,8 +170,12 @@ SYSCALL_DEFINE3(s390_pci_mmio_write, unsigned long, mmio_addr,
goto out_unlock_mmap;
ret = follow_pte(vma, mmio_addr, &ptep, &ptl);
- if (ret)
- goto out_unlock_mmap;
+ if (ret) {
+ fixup_user_fault(current->mm, mmio_addr, FAULT_FLAG_WRITE, NULL);
+ ret = follow_pte(vma, mmio_addr, &ptep, &ptl);
+ if (ret)
+ goto out_unlock_mmap;
+ }
io_addr = (void __iomem *)((pte_pfn(*ptep) << PAGE_SHIFT) |
(mmio_addr & ~PAGE_MASK));
@@ -305,12 +309,16 @@ SYSCALL_DEFINE3(s390_pci_mmio_read, unsigned long, mmio_addr,
if (!(vma->vm_flags & (VM_IO | VM_PFNMAP)))
goto out_unlock_mmap;
ret = -EACCES;
- if (!(vma->vm_flags & VM_WRITE))
+ if (!(vma->vm_flags & VM_READ))
goto out_unlock_mmap;
ret = follow_pte(vma, mmio_addr, &ptep, &ptl);
- if (ret)
- goto out_unlock_mmap;
+ if (ret) {
+ fixup_user_fault(current->mm, mmio_addr, 0, NULL);
+ ret = follow_pte(vma, mmio_addr, &ptep, &ptl);
+ if (ret)
+ goto out_unlock_mmap;
+ }
io_addr = (void __iomem *)((pte_pfn(*ptep) << PAGE_SHIFT) |
(mmio_addr & ~PAGE_MASK));
Ughh, I think I just stumbled over a problem with this. This is a
failing lock held assertion via __is_vma_write_locked() in
remap_pfn_range_notrack() but I'm not sure yet what exactly causes this
[ 67.338855] ------------[ cut here ]------------
[ 67.338865] WARNING: CPU: 15 PID: 2056 at include/linux/rwsem.h:85 remap_pfn_range_notrack+0x596/0x5b0
[ 67.338874] Modules linked in: <--- 8< --->
[ 67.338931] CPU: 15 PID: 2056 Comm: vfio-test Not tainted 6.10.0-rc1-pci-pfault-00004-g193e3a513cee #5
[ 67.338934] Hardware name: IBM 3931 A01 701 (LPAR)
[ 67.338935] Krnl PSW : 0704c00180000000 000003e54c9730ea (remap_pfn_range_notrack+0x59a/0x5b0)
[ 67.338940] R:0 T:1 IO:1 EX:1 Key:0 M:1 W:0 P:0 AS:3 CC:0 PM:0 RI:0 EA:3
[ 67.338944] Krnl GPRS: 0000000000000100 000003655915fb78 000002d80b9a5928 000003ff7fa00000
[ 67.338946] 0004008000000000 0000000000004000 0000000000000711 000003ff7fa04000
[ 67.338948] 000002d80c533f00 000002d800000100 000002d81bbe6c28 000002d80b9a5928
[ 67.338950] 000003ff7fa00000 000002d80c533f00 000003e54c973120 000003655915fab0
[ 67.338956] Krnl Code: 000003e54c9730de: a708ffea lhi %r0,-22
000003e54c9730e2: a7f4fff6 brc 15,000003e54c9730ce
#000003e54c9730e6: af000000 mc 0,0
>000003e54c9730ea: a7f4fd6e brc 15,000003e54c972bc6
000003e54c9730ee: af000000 mc 0,0
000003e54c9730f2: af000000 mc 0,0
000003e54c9730f6: 0707 bcr 0,%r7
000003e54c9730f8: 0707 bcr 0,%r7
[ 67.339025] Call Trace:
[ 67.339027] [<000003e54c9730ea>] remap_pfn_range_notrack+0x59a/0x5b0
[ 67.339032] [<000003e54c973120>] remap_pfn_range+0x20/0x30
[ 67.339035] [<000003e4cce5396c>] vfio_pci_mmap_fault+0xec/0x1d0 [vfio_pci_core]
[ 67.339043] [<000003e54c977240>] handle_mm_fault+0x6b0/0x25a0
[ 67.339046] [<000003e54c966328>] fixup_user_fault+0x138/0x310
[ 67.339048] [<000003e54c63a91c>] __s390x_sys_s390_pci_mmio_read+0x28c/0x3a0
[ 67.339051] [<000003e54c5e200a>] do_syscall+0xea/0x120
[ 67.339055] [<000003e54d5f9954>] __do_syscall+0x94/0x140
[ 67.339059] [<000003e54d611020>] system_call+0x70/0xa0
[ 67.339063] Last Breaking-Event-Address:
[ 67.339065] [<000003e54c972bc2>] remap_pfn_range_notrack+0x72/0x5b0
[ 67.339067] ---[ end trace 0000000000000000 ]---
This has me a bit confused so far as __is_vma_write_locked() checks
mmap_assert_write_locked(vma->vm_mm) but most other users of
fixup_user_fault() hold mmap_read_lock() just like this code and
clearly in the non page fault case we only need the read lock.
This is likely the
vm_flags_set()->vma_start_write(vma)->__is_vma_write_locked()
which checks mmap_assert_write_locked().
Setting VMA flags would be racy with the mmap lock in read mode.
remap_pfn_range() documents: "this is only safe if the mm semaphore is
held when called." which doesn't spell out if it needs to be held in
write mode (which I think it does) :)
My best guess is: if you are using remap_pfn_range() from a fault
handler (not during mmap time) you are doing something wrong, that's why
you get that report.
vmf_insert_pfn() and friends might be better alternatives, that make
sure that the VMA already received the proper VMA flags at mmap time.
And it gets weirder, as I could have sworn that I properly tested this
on v1, I retested with v1 (tags/sent/vfio_pci_mmap-v1 on my
git.kernel.org/niks and based on v6.9) and there I don't get the above
warning. I also made sure that it's not caused by my change to
"current->mm" for v2. But I'm also not hitting the checks David moved
into follow_pte() so yeah not sure what's going on here.
You mean the mmap_assert_locked()? Yeah, that only checks if you have it
in read mode, but not in write mode.
--
Cheers,
David / dhildenb