Re: [syzbot] [netfs?] BUG: unable to handle kernel NULL pointer dereference in netfs_unbuffered_write

[syzbot]

Hello,

syzbot tried to test the proposed patch but the build/boot failed:

lost connection to test machine



qemu-system-x86_64: ahci: PRDT length for NCQ command (0x0) is smaller than the requested size (0xec000)
qemu-system-x86_64: ahci: PRDT length for NCQ command (0x0) is smaller than the requested size (0x372000)
qemu-system-x86_64: ahci: PRDT length for NCQ command (0x0) is smaller than the requested size (0x2b4000)
Warning: Permanently added '[localhost]:9553' (ED25519) to the list of known hosts.
[   60.013321][   T40] audit: type=1400 audit(1772846336.792:62): avc:  denied  { execute } for  pid=5928 comm="sh" name="syz-execprog" dev="sda1" ino=2020 scontext=root:sysadm_r:sysadm_t tcontext=root:object_r:etc_runtime_t tclass=file permissive=1
[   60.020535][   T40] audit: type=1400 audit(1772846336.802:63): avc:  denied  { execute_no_trans } for  pid=5928 comm="sh" path="/syz-execprog" dev="sda1" ino=2020 scontext=root:sysadm_r:sysadm_t tcontext=root:object_r:etc_runtime_t tclass=file permissive=1
2026/03/07 01:18:58 parsed 1 programs
[   61.787218][   T40] audit: type=1400 audit(1772846338.562:64): avc:  denied  { node_bind } for  pid=5928 comm="syz-execprog" saddr=::1 scontext=root:sysadm_r:sysadm_t tcontext=system_u:object_r:node_t tclass=tcp_socket permissive=1
[   64.314024][   T40] audit: type=1400 audit(1772846341.092:65): avc:  denied  { mounton } for  pid=5937 comm="syz-executor" path="/syzcgroup/unified" dev="sda1" ino=2023 scontext=root:sysadm_r:sysadm_t tcontext=root:object_r:root_t tclass=dir permissive=1
[   64.325364][   T40] audit: type=1400 audit(1772846341.102:66): avc:  denied  { mount } for  pid=5937 comm="syz-executor" name="/" dev="cgroup2" ino=1 scontext=root:sysadm_r:sysadm_t tcontext=system_u:object_r:cgroup_t tclass=filesystem permissive=1
[   64.327352][ T5937] cgroup: Unknown subsys name 'net'
[   64.338664][   T40] audit: type=1400 audit(1772846341.112:67): avc:  denied  { unmount } for  pid=5937 comm="syz-executor" scontext=root:sysadm_r:sysadm_t tcontext=system_u:object_r:cgroup_t tclass=filesystem permissive=1
[   64.492218][ T5937] cgroup: Unknown subsys name 'cpuset'
[   64.497527][ T5937] cgroup: Unknown subsys name 'rlimit'
[   64.697609][   T40] audit: type=1400 audit(1772846341.472:68): avc:  denied  { setattr } for  pid=5937 comm="syz-executor" name="raw-gadget" dev="devtmpfs" ino=849 scontext=root:sysadm_r:sysadm_t tcontext=system_u:object_r:device_t tclass=chr_file permissive=1
[   64.709106][   T40] audit: type=1400 audit(1772846341.482:69): avc:  denied  { create } for  pid=5937 comm="syz-executor" scontext=root:sysadm_r:sysadm_t tcontext=root:sysadm_r:sysadm_t tclass=netlink_generic_socket permissive=1
[   64.717475][   T40] audit: type=1400 audit(1772846341.482:70): avc:  denied  { write } for  pid=5937 comm="syz-executor" scontext=root:sysadm_r:sysadm_t tcontext=root:sysadm_r:sysadm_t tclass=netlink_generic_socket permissive=1
[   64.726303][   T40] audit: type=1400 audit(1772846341.482:71): avc:  denied  { read } for  pid=5937 comm="syz-executor" scontext=root:sysadm_r:sysadm_t tcontext=root:sysadm_r:sysadm_t tclass=netlink_generic_socket permissive=1
[   76.424366][ T1416] ieee802154 phy0 wpan0: encryption failed: -22
[   76.427236][ T1416] ieee802154 phy1 wpan1: encryption failed: -22
[   86.668293][   T29] cfg80211: failed to load regulatory.db
[  118.059383][ T1113] ata1.00: NCQ disabled due to excessive errors
[  118.062114][ T1113] ata1.00: exception Emask 0x0 SAct 0x40000120 SErr 0x0 action 0x6 frozen
[  118.065797][ T1113] ata1.00: failed command: WRITE FPDMA QUEUED
[  118.068375][ T1113] ata1.00: cmd 61/90:28:36:21:05/1b:00:00:00:00/40 tag 5 ncq dma 3612672 ou
[  118.068375][ T1113]          res 40/00:00:00:00:00/00:00:00:00:00/00 Emask 0x4 (timeout)
[  118.075829][ T1113] ata1.00: status: { DRDY }
[  118.077372][ T1113] ata1.00: failed command: WRITE FPDMA QUEUED
[  118.079529][ T1113] ata1.00: cmd 61/a0:40:36:61:05/15:00:00:00:00/40 tag 8 ncq dma 2834432 ou
[  118.079529][ T1113]          res 40/00:00:00:00:00/00:00:00:00:00/00 Emask 0x4 (timeout)
[  118.084906][ T1113] ata1.00: status: { DRDY }
[  118.086422][ T1113] ata1.00: failed command: WRITE FPDMA QUEUED
[  118.088377][ T1113] ata1.00: cmd 61/60:f0:76:4a:04/07:00:00:00:00/40 tag 30 ncq dma 966656 out
[  118.088377][ T1113]          res 40/00:00:00:00:00/00:00:00:00:00/00 Emask 0x4 (timeout)
[  118.093826][ T1113] ata1.00: status: { DRDY }
[  118.095370][ T1113] ata1: hard resetting link
[  118.414125][ T1113] ata1: SATA link up 1.5 Gbps (SStatus 113 SControl 300)
[  118.417532][ T1113] ata1.00: configured for UDMA/100
[  118.419565][ T1113] ata1: EH complete
qemu-system-x86_64: hw/ide/core.c:934: ide_dma_cb: Assertion `prep_size >= 0 && prep_size <= n * 512' failed.
Connection to localhost closed by remote host.


syzkaller build log:
go env (err=<nil>)
AR='ar'
CC='gcc'
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_ENABLED='1'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
CXX='g++'
GCCGO='gccgo'
GO111MODULE='auto'
GOAMD64='v1'
GOARCH='amd64'
GOAUTH='netrc'
GOBIN=''
GOCACHE='/syzkaller/.cache/go-build'
GOCACHEPROG=''
GODEBUG=''
GOENV='/syzkaller/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFIPS140='off'
GOFLAGS=''
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build632602998=/tmp/go-build -gno-record-gcc-switches'
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMOD='/syzkaller/jobs/linux/gopath/src/github.com/google/syzkaller/go.mod'
GOMODCACHE='/syzkaller/jobs/linux/gopath/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/syzkaller/jobs/linux/gopath'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/usr/local/go'
GOSUMDB='sum.golang.org'
GOTELEMETRY='local'
GOTELEMETRYDIR='/syzkaller/.config/go/telemetry'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/usr/local/go/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.24.4'
GOWORK=''
PKG_CONFIG='pkg-config'

git status (err=<nil>)
HEAD detached at d20b04c80a0
nothing to commit, working tree clean


tput: No value for $TERM and no -T specified
tput: No value for $TERM and no -T specified
Makefile:31: run command via tools/syz-env for best compatibility, see:
Makefile:32: https://github.com/google/syzkaller/blob/master/docs/contributing.md#using-syz-env
go list -f '{{.Stale}}' -ldflags="-s -w -X github.com/google/syzkaller/prog.GitRevision=d20b04c80a01e68026299511a6ba77cc67a198f5 -X github.com/google/syzkaller/prog.gitRevisionDate=20260305-101922"  ./sys/syz-sysgen | grep -q false || go install -ldflags="-s -w -X github.com/google/syzkaller/prog.GitRevision=d20b04c80a01e68026299511a6ba77cc67a198f5 -X github.com/google/syzkaller/prog.gitRevisionDate=20260305-101922"  ./sys/syz-sysgen
make .descriptions
tput: No value for $TERM and no -T specified
tput: No value for $TERM and no -T specified
Makefile:31: run command via tools/syz-env for best compatibility, see:
Makefile:32: https://github.com/google/syzkaller/blob/master/docs/contributing.md#using-syz-env
bin/syz-sysgen
touch .descriptions
GOOS=linux GOARCH=amd64 go build -ldflags="-s -w -X github.com/google/syzkaller/prog.GitRevision=d20b04c80a01e68026299511a6ba77cc67a198f5 -X github.com/google/syzkaller/prog.gitRevisionDate=20260305-101922"  -o ./bin/linux_amd64/syz-execprog github.com/google/syzkaller/tools/syz-execprog
mkdir -p ./bin/linux_amd64
g++ -o ./bin/linux_amd64/syz-executor executor/executor.cc \
	-m64 -O2 -pthread -Wall -Werror -Wparentheses -Wunused-const-variable -Wframe-larger-than=16384 -Wno-stringop-overflow -Wno-array-bounds -Wno-format-overflow -Wno-unused-but-set-variable -Wno-unused-command-line-argument -static-pie -std=c++17 -I. -Iexecutor/_include   -DGOOS_linux=1 -DGOARCH_amd64=1 \
	-DHOSTGOOS_linux=1 -DGIT_REVISION=\"d20b04c80a01e68026299511a6ba77cc67a198f5\"
/usr/bin/ld: /tmp/ccWzPJKl.o: in function `Connection::Connect(char const*, char const*)':
executor.cc:(.text._ZN10Connection7ConnectEPKcS1_[_ZN10Connection7ConnectEPKcS1_]+0x386): warning: Using 'gethostbyname' in statically linked applications requires at runtime the shared libraries from the glibc version used for linking
./tools/check-syzos.sh 2>/dev/null



Tested on:

commit:         325a118c Merge tag 'pci-v7.0-fixes-3' of git://git.ker..
git tree:       upstream
kernel config:  https://syzkaller.appspot.com/x/.config?x=976ba5a93c4add9e
dashboard link: https://syzkaller.appspot.com/bug?extid=7227db0fbac9f348dba0
compiler:       gcc (Debian 14.2.0-19) 14.2.0, GNU ld (GNU Binutils for Debian) 2.44
patch:          https://syzkaller.appspot.com/x/patch.diff?x=12486b5a580000