** Also affects: linux (Ubuntu Yakkety)
Importance: Undecided
Status: New
** Also affects: linux (Ubuntu Zesty)
Importance: Undecided
Status: New
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https://bugs.launchpad.net/bugs/1672819
Title:
exec'ing a setuid binary from a threaded program sometimes fails to
setuid
Status in Linux:
Unknown
Status in linux package in Ubuntu:
Fix Committed
Status in linux source package in Xenial:
In Progress
Status in linux source package in Yakkety:
New
Status in linux source package in Zesty:
New
Bug description:
== SRU REQUEST XENIAL, YAKKETY, ZESTY ==
Due to two race conditions in check_unsafe_exec(), exec'ing a setuid
binary from a threaded program sometimes fails to setuid.
== Fix ==
Sauce patch for Xenial, Yakkety + Zesty:
https://lists.ubuntu.com/archives/kernel-team/2017-May/084102.html
This fix re-executes the unsafe check if there is a discrepancy
between the expected fs count and the found count during the racy
window during thread exec or exit. This re-check occurs very
infrequently and saves a lot of addition locking on per thread
structures that would make performance of fork/exec/exit prohibitively
expensive.
== Test case ==
See the example C code in the patch, https://lists.ubuntu.com/archives
/kernel-team/2017-May/084102.html
Run the test code as follows: for i in $(seq 1000); do ./a; done
With the patch, no messages are emitted, without the patch, one sees a
message:
"Failed, got euid 1000 (expecting 0)"
..which shows the setuid program failed the check_unsafe_exec()
because of the race.
== Regression potential ==
breaking existing safe exec semantics.
====================
This can be reproduced with
https://gist.github.com/chipaca/806c90d96c437444f27f45a83d00a813
With that, and go 1.8, if you run “make” and then
for i in `seq 99`; do ./a_go; done
you'll see a variable number of ”GOT 1000” (or whatever your user id
is). If you don't, add one or two more 9s on there.
That's a simple go reproducer. You can also use “a_p” instead of
“a_go” to see one that only uses pthreads. “a_c” is a C version that
does *not* reproduce the issue.
But it's not pthreads: if in a_go.go you comment out the “import "C"”,
you'll still see the “GOT 1000” messages, in a static binary that uses
no pthreads, just clone(2). You'll also see a bunch of warnings
because it's not properly handling an EAGAIN from clone, but that's
unrelated.
If you pin the process to a single thread using taskset, you don't get
the issue from a_go; a_p continues to reproduce the issue. In some
virtualized environments we haven't been able to reproduce the issue
either (e.g. some aws instances), but kvm works (you need -smp to see
the issue from a_go).
ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: linux-image-4.4.0-64-generic 4.4.0-64.85
ProcVersionSignature: Ubuntu 4.4.0-64.85-generic 4.4.44
Uname: Linux 4.4.0-64-generic x86_64
NonfreeKernelModules: zfs zunicode zcommon znvpair zavl
ApportVersion: 2.20.1-0ubuntu2.5
Architecture: amd64
AudioDevicesInUse:
USER PID ACCESS COMMAND
/dev/snd/pcmC0D0p: john 2354 F...m pulseaudio
/dev/snd/controlC0: john 2354 F.... pulseaudio
CurrentDesktop: Unity
Date: Tue Mar 14 17:17:23 2017
HibernationDevice: RESUME=UUID=b9fd155b-dcbe-4337-ae77-6daa6569beaf
InstallationDate: Installed on 2014-04-27 (1051 days ago)
InstallationMedia: Ubuntu 14.04 LTS "Trusty Tahr" - Release amd64 (20140417)
MachineType: Dell Inc. Latitude E6510
ProcFB: 0 inteldrmfb
ProcKernelCmdLine: BOOT_IMAGE=/vmlinuz-4.4.0-64-generic
root=/dev/mapper/ubuntu--vg-root ro enable_mtrr_cleanup mtrr_spare_reg_nr=8
mtrr_gran_size=32M mtrr_chunk_size=32M quiet splash
RelatedPackageVersions:
linux-restricted-modules-4.4.0-64-generic N/A
linux-backports-modules-4.4.0-64-generic N/A
linux-firmware 1.157.8
SourcePackage: linux
SystemImageInfo: Error: command ['system-image-cli', '-i'] failed with exit
code 2:
UpgradeStatus: Upgraded to xenial on 2015-06-18 (634 days ago)
dmi.bios.date: 12/05/2013
dmi.bios.vendor: Dell Inc.
dmi.bios.version: A16
dmi.board.vendor: Dell Inc.
dmi.chassis.type: 9
dmi.chassis.vendor: Dell Inc.
dmi.modalias:
dmi:bvnDellInc.:bvrA16:bd12/05/2013:svnDellInc.:pnLatitudeE6510:pvr0001:rvnDellInc.:rn:rvr:cvnDellInc.:ct9:cvr:
dmi.product.name: Latitude E6510
dmi.product.version: 0001
dmi.sys.vendor: Dell Inc.
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