Re: [BUG] virtio-fs: Corruption when running binaries from virtiofsd-backed fs

From: Erik Schilling
Date: Wed Sep 13 2023 - 10:01:06 EST

CCing a few more people as suggested by stefanha on #qemu.

On Wed Sep 13, 2023 at 8:18 AM CEST, Erik Schilling wrote:
> On Fri Sep 1, 2023 at 12:37 PM CEST, Erik Schilling wrote:
> > On Wed Aug 30, 2023 at 10:20 AM CEST, Erik Schilling wrote:
> > > Hi all!
> > >
> > > Some days ago I posted to #virtiofs:matrix.org, describing that I am
> > > observing what looks like a corruption when executing programs from a
> > > virtiofs-based filesystem.
> > >
> > > Over the last few days I spent more time drilling into the problem.
> > > This is an attempt at summarizing my findings in order to see what other
> > > people think about this.
> > >
> > > When running binaries mounted from virtiofs they may either: fail with a
> > > segfault, fail with badaddr, get stuck or - sometimes - succeed.
> > >
> > > Environment:
> > > Host: Fedora 38 running 6.4.11-200.fc38.x86_64
> > > Guest: Yocto-based image: 6.4.9-yocto-standard, aarch64
> > > virtiofsd: latest main + some debug prints [1]
> > > QEMU: built from recent git [2]
> > >
> > > virtiofsd invocation:
> > > RUST_LOG="debug" ./virtiofsd --seccomp=none --sandbox=none \
> > > --socket-path "fs.sock0" --shared-dir $PWD/share-dir/ --cache=never
> > >
> > > QEMU invocation:
> > > ~/projects/qemu/build/qemu-system-aarch64 -kernel Image -machine virt \
> > > -cpu cortex-a57 \
> > > -serial mon:stdio \
> > > -device virtio-net-pci,netdev=net0 \
> > > -netdev user,id=net0,hostfwd=tcp::2223-:22 \
> > > -display none -m 2048 -smp 4 \
> > > -object memory-backend-memfd,id=mem,size=2048M,share=on \
> > > -numa node,memdev=mem \
> > > -hda trs-overlay-guest.qcow2 \
> > > -chardev socket,id=char0,path="fs.sock0" \
> > > -device vhost-user-fs-pci,queue-size=1024,chardev=char0,tag=/dev/root \
> > > -append 'root=/dev/vda2 ro log_buf_len=8M'
> > >
> > > I figured that launching virtiofsd with --cache=always masks the
> > > problem. Therefore, I set --cache=never, but I think I observed no
> > > difference compared to the default setting (auto).
> > >
> > > Adding logging to virtiofsd and kernel _feeled_ like it made the problem
> > > harder to reproduce - leaving me with the impression that some race is
> > > happening on somewhere.
> > >
> > > Trying to rule out that virtiofsd is returning corrupted data, I added
> > > some logging and hashsum calculation hacks to it [1]. The hashes check
> > > out across multiple accesses and the order and kind of queued messages
> > > is exactly the same in both the error case and crash case. fio was also
> > > unable to find any errors with a naive job description [3].
> > >
> > > Next, I tried to capture info on the guest side. This became a bit
> > > tricky since the crashes became pretty rare once I followed a fixed
> > > pattern of starting log capture, running perf and trying to reproduce
> > > the problem. Ultimately, I had the most consistent results with
> > > immediately running a program twice:
> > >
> > > /mnt/ld-linux-aarch64.so.1 /mnt/ls.coreutils /; \
> > > /mnt/ld-linux-aarch64.so.1 /mnt/ls.coreutils /
> > >
> > > (/mnt being the virtiofs mount)
> > >
> > > For collecting logs, I made a hack to the guest kernel in order to dump
> > > the page content after receiving the virtiofs responses [4]. Reproducing
> > > the problem with this, leaves me with logs that seem to suggest that
> > > virtiofsd is returning identical content, but the guest kernel seems to
> > > receive differing pages:
> > >
> > > good-kernel [5]:
> > > kernel: virtio_fs_wake_pending_and_unlock: opcode 3 unique 0x312 nodeid 0x1 in.len 56 out.len 104
> > > kernel: virtiofs virtio1: virtio_fs_vq_done requests.0
> > > kernel: virtio_fs_wake_pending_and_unlock: opcode 1 unique 0x314 nodeid 0x1 in.len 53 out.len 128
> > > kernel: virtiofs virtio1: virtio_fs_vq_done requests.0
> > > kernel: virtio_fs_wake_pending_and_unlock: opcode 3 unique 0x316 nodeid 0x29 in.len 56 out.len 104
> > > kernel: virtiofs virtio1: virtio_fs_vq_done requests.0
> > > kernel: virtio_fs_wake_pending_and_unlock: opcode 14 unique 0x318 nodeid 0x29 in.len 48 out.len 16
> > > kernel: virtiofs virtio1: virtio_fs_vq_done requests.0
> > > kernel: virtio_fs_wake_pending_and_unlock: opcode 15 unique 0x31a nodeid 0x29 in.len 80 out.len 832
> > > kernel: virtiofs virtio1: virtio_fs_vq_done requests.0
> > > kernel: virtio_fs: page: 000000006996d520
> > > kernel: virtio_fs: to: 00000000de590c14
> > > kernel: virtio_fs rsp:00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
> > > kernel: virtio_fs rsp:00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
> > > kernel: virtio_fs rsp:00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
> > > kernel: virtio_fs rsp:00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
> > > [...]
> > >
> > > bad-kernel [6]:
> > > kernel: virtio_fs_wake_pending_and_unlock: opcode 3 unique 0x162 nodeid 0x1 in.len 56 out.len 104
> > > kernel: virtiofs virtio1: virtio_fs_vq_done requests.0
> > > kernel: virtio_fs_wake_pending_and_unlock: opcode 1 unique 0x164 nodeid 0x1 in.len 53 out.len 128
> > > kernel: virtiofs virtio1: virtio_fs_vq_done requests.0
> > > kernel: virtio_fs_wake_pending_and_unlock: opcode 3 unique 0x166 nodeid 0x16 in.len 56 out.len 104
> > > kernel: virtiofs virtio1: virtio_fs_vq_done requests.0
> > > kernel: virtio_fs_wake_pending_and_unlock: opcode 14 unique 0x168 nodeid 0x16 in.len 48 out.len 16
> > > kernel: virtiofs virtio1: virtio_fs_vq_done requests.0
> > > kernel: virtio_fs_wake_pending_and_unlock: opcode 15 unique 0x16a nodeid 0x16 in.len 80 out.len 832
> > > kernel: virtiofs virtio1: virtio_fs_vq_done requests.0
> > > kernel: virtio_fs: page: 000000006ce9a559
> > > kernel: virtio_fs: to: 000000007ae8b946
> > > kernel: virtio_fs rsp:00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
> > > kernel: virtio_fs rsp:00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
> > > kernel: virtio_fs rsp:80 40 de c8 ff ff 00 00 cc 2b 62 ae ff ff 00 00 .@.......+b.....
> > > kernel: virtio_fs rsp:02 4e de c8 ff ff 00 00 00 00 00 00 00 00 00 00 .N..............
> > > [...]
> > >
> > > When looking at the corresponding output from virtiofsd, it claims to
> > > have returned identical data:
> > >
> > > good-virtiofsd [7]:
> > > [DEBUG virtiofsd::server] Received request: opcode=Read (15), inode=41, unique=794, pid=481
> > > [src/server.rs:618] r.read_obj().map_err(Error::DecodeMessage)? = ReadIn {
> > > fh: 31,
> > > offset: 0,
> > > size: 832,
> > > read_flags: 2,
> > > lock_owner: 6838554705639967244,
> > > flags: 131072,
> > > padding: 0,
> > > }
> > > [src/file_traits.rs:161] hash = 2308490450751364994
> > > [DEBUG virtiofsd::server] Replying OK, header: OutHeader { len: 848, error: 0, unique: 794 }
> > >
> > > bad-virtiofsd [8]:
> > > [DEBUG virtiofsd::server] Received request: opcode=Read (15), inode=22, unique=362, pid=406
> > > [src/server.rs:618] r.read_obj().map_err(Error::DecodeMessage)? = ReadIn {
> > > fh: 12,
> > > offset: 0,
> > > size: 832,
> > > read_flags: 2,
> > > lock_owner: 6181120926258395554,
> > > flags: 131072,
> > > padding: 0,
> > > }
> > > [src/file_traits.rs:161] hash = 2308490450751364994
> > > [DEBUG virtiofsd::server] Replying OK, header: OutHeader { len: 848, error: 0, unique: 362 }
> > >
> > > The "corruption" only seems to happen in this one page, all other pages
> > > are identical between runs (except that the bad run terminates earlier).
> > >
> > > What do the experts think here? To me it feels a bit like some kind of
> > > corruption is going on. Or am I misinterpreting things here?
> > >
> > > Which further analysis steps would you suggest?
> > >
> > >
> > > Further notes:
> > >
> > > After collecting the above results, I realized that running the guest
> > > with -smp 1 makes the problems a lot worse. So maybe that is a better
> > > choice when trying to reproduce it.
> > >
> > > Repo with my scripts is available at:
> > > https://git.codelinaro.org/erik_schilling/jira-orko-65-bootstrap-k3s-config/
> > >
> > > The scripts are just quick and dirty implementations and are not
> > > particulary portable.
> >
> > Summary of my testing during the last few days:
> >
> > Testing with KCSAN revealed a few cases that look like missing READ_ONCE
> > annotations (will send patches separately). But nothing of that was
> > related to the immediate problem. I tested instrument_read() and another
> > round of logging with a delay to virtio_fs_request_complete. It looks
> > like the buffer get corrupted before entering that function. KCSAN
> > or manual sleeps + prints did not show any corruption while in that
> > function.
> >
> > KASAN did not report any issues.
> >
> > Patching virtiofsd to do an additional copy and going through rust-vmm's
> > .copy_to() function did not change the behaviour.
> >
> > I will mostly be off next week, will continue analysis afterwards. Happy
> > to hear about suggestions of other things to try :).
>
> Back from a week of vacation...
>
> Summary of what was discussed on #virtiofs:matrix.org:
>
> The issue only seems to happen in QEMU TCG scenarios (I tested aarch64
> and x86_64 on x86_64, wizzard on Matrix tested arm32).
>
> CCing qemu-devel. Maybe someone has some hints on where to focus the
> debugging efforts?
>
> I am trying to build a complex monster script of tracing the relevant
> addresses in order to figure out whether the guest or host does the
> writes. But I am happy to hear about more clever ideas :).

After hearing about investigations of bugs in other virtio scenarios
that seem to be caused by QEMU [9], I tested some older QEMU versions.

Indeed, a882b5712373171d3bd53cd82ddab4453ddef468 did not show the buggy
behaviour. So I did a bisect:

git bisect start
# status: waiting for both good and bad commits
# good: [a882b5712373171d3bd53cd82ddab4453ddef468] Revert "virtio: introduce macro IRTIO_CONFIG_IRQ_IDX"
git bisect good a882b5712373171d3bd53cd82ddab4453ddef468
# status: waiting for bad commit, 1 good commit known
# bad: [9ef497755afc252fb8e060c9ea6b0987abfd20b6] Merge tag 'pull-vfio-20230911' of https://github.com/legoater/qemu into staging
git bisect bad 9ef497755afc252fb8e060c9ea6b0987abfd20b6
# skip: [3ba5fe46ea4456a16e2f47ab8e75943b54879c4e] Merge tag 'mips-20221108' of https://github.com/philmd/qemu into staging
git bisect skip 3ba5fe46ea4456a16e2f47ab8e75943b54879c4e
# skip: [ade760a2f63804b7ab1839fbc3e5ddbf30538718] Merge tag 'pull-request-2022-11-08' of https://gitlab.com/thuth/qemu into staging
git bisect skip ade760a2f63804b7ab1839fbc3e5ddbf30538718
# good: [ad2ca2e3f762b0cb98eb976002569795b270aef1] target/xtensa: Drop tcg_temp_free
git bisect good ad2ca2e3f762b0cb98eb976002569795b270aef1
# bad: [19a720b74fde7e859d19f12c66a72e545947a657] Merge tag 'tracing-pull-request' of https://gitlab.com/stefanha/qemu into staging
git bisect bad 19a720b74fde7e859d19f12c66a72e545947a657
# bad: [29d9efca16080211f107b540f04d1ed3c12c63b0] arm/Kconfig: Do not build TCG-only boards on a KVM-only build
git bisect bad 29d9efca16080211f107b540f04d1ed3c12c63b0
# good: [9636e513255362c4a329e3e5fb2c97dab3c5ce47] Merge tag 'misc-next-pull-request' of https://gitlab.com/berrange/qemu into staging
git bisect good 9636e513255362c4a329e3e5fb2c97dab3c5ce47
# bad: [45608654aa63ca2b311d6cb761e1522f2128e00e] Merge tag 'pull-tpm-2023-04-20-1' of https://github.com/stefanberger/qemu-tpm into staging
git bisect bad 45608654aa63ca2b311d6cb761e1522f2128e00e
# good: [1ff4a81bd3efb207992f1da267886fe0c4df764f] tcg: use QTree instead of GTree
git bisect good 1ff4a81bd3efb207992f1da267886fe0c4df764f
# bad: [9ed98cae151368cc89c4bb77c9f325f7185e8f09] block-backend: ignore inserted state in blk_co_nb_sectors
git bisect bad 9ed98cae151368cc89c4bb77c9f325f7185e8f09
# good: [c8cb603293fd329f2a62ade76ec9de3f462fc5c3] tests/avocado: Test Xen guest support under KVM
git bisect good c8cb603293fd329f2a62ade76ec9de3f462fc5c3
# bad: [64f1c63d87208e28e8e38c4ab514ada1728960ef] Merge tag 'pull_error_handle_fix_use_after_free.v1' of https://github.com/stefanberger/qemu-tpm into staging
git bisect bad 64f1c63d87208e28e8e38c4ab514ada1728960ef
# good: [8a712df4d4d736b7fe6441626677bfd271d95b15] Merge tag 'pull-for-8.0-040423-2' of https://gitlab.com/stsquad/qemu into staging
git bisect good 8a712df4d4d736b7fe6441626677bfd271d95b15
# bad: [7d0334e49111787ae19fbc8d29ff6e7347f0605e] Merge tag 'pull-tcg-20230404' of https://gitlab.com/rth7680/qemu into staging
git bisect bad 7d0334e49111787ae19fbc8d29ff6e7347f0605e
# bad: [3371802fba3f7be4465f8a5e5777d43d556676ef] accel/tcg: Fix jump cache set in cpu_exec_loop
git bisect bad 3371802fba3f7be4465f8a5e5777d43d556676ef
# good: [6cda41daa2162b8e1048124655ba02a8c2b762b4] Revert "linux-user/arm: Take more care allocating commpage"
git bisect good 6cda41daa2162b8e1048124655ba02a8c2b762b4
# skip: [c83574392e0af108a643347712564f6749906413] accel/tcg: Fix overwrite problems of tcg_cflags
git bisect skip c83574392e0af108a643347712564f6749906413
# only skipped commits left to test
# possible first bad commit: [3371802fba3f7be4465f8a5e5777d43d556676ef] accel/tcg: Fix jump cache set in cpu_exec_loop
# possible first bad commit: [c83574392e0af108a643347712564f6749906413] accel/tcg: Fix overwrite problems of tcg_cflags

I had an inclusive test in the end where c83574392e did not yield in me
being able to start the VM.

Whether one of these contains a bug or whether only new behaviour of
QEMU revealed a bug somewhere else is of course still to be figured out.

[9] https://gitlab.com/qemu-project/qemu/-/issues/1866

- Erik

>
> - Erik
>
> >
> > Good weekend,
> >
> > - Erik
> >
> >
> > >
> > > - Erik
> > >
> > > [1] https://gitlab.com/ablu/virtiofsd/-/commit/18fd0c1849e15bc55fbdd6e1f169801b2b03da1f
> > > [2] https://gitlab.com/qemu-project/qemu/-/commit/50e7a40af372ee5931c99ef7390f5d3d6fbf6ec4
> > > [3] https://git.codelinaro.org/erik_schilling/jira-orko-65-bootstrap-k3s-config/-/blob/397a6310dea35973025e3d61f46090bf0c092762/share-dir/write-and-verify-mmap.fio
> > > [4] https://github.com/Ablu/linux/commit/3880b9f8affb01aeabb0a04fe76ad7701dc0bb95
> > > [5] Line 12923: https://git.codelinaro.org/erik_schilling/jira-orko-65-bootstrap-k3s-config/-/blob/main/logs/2023-08-29%2013%3A42%3A35%2B02%3A00/good-drop-bad-1.txt
> > > [6] Line 12923: https://git.codelinaro.org/erik_schilling/jira-orko-65-bootstrap-k3s-config/-/blob/main/logs/2023-08-29%2013%3A42%3A35%2B02%3A00/good-bad-1.txt
> > > [7] https://git.codelinaro.org/erik_schilling/jira-orko-65-bootstrap-k3s-config/-/blob/main/logs/2023-08-29%2013%3A42%3A35%2B02%3A00/virtiofsd.txt#L2538-2549
> > > [8] https://git.codelinaro.org/erik_schilling/jira-orko-65-bootstrap-k3s-config/-/blob/main/logs/2023-08-29%2013%3A42%3A35%2B02%3A00/virtiofsd.txt#L1052-1063