9p/xen: protect xen_9pfs_front_free against concurrent calls

Overview

In the Linux kernel, the following vulnerability has been resolved:

9p/xen: protect xen_9pfs_front_free against concurrent calls

The xenwatch thread can race with other back-end change notifications and call xen_9pfs_front_free() twice, hitting the observed general protection fault due to a double-free. Guard the teardown path so only one caller can release the front-end state at a time, preventing the crash.

This is a fix for the following double-free:

[ 27.052347] Oops: general protection fault, probably for non-canonical address 0x6b6b6b6b6b6b6b6b: 0000 [#1] SMP DEBUG_PAGEALLOC NOPTI [ 27.052357] CPU: 0 UID: 0 PID: 32 Comm: xenwatch Not tainted 6.18.0-02087-g51ab33fc0a8b-dirty #60 PREEMPT(none) [ 27.052363] RIP: e030:xen_9pfs_front_free+0x1d/0x150 [ 27.052368] Code: 90 90 90 90 90 90 90 90 90 90 90 90 90 41 55 41 54 55 48 89 fd 48 c7 c7 48 d0 92 85 53 e8 cb cb 05 00 48 8b 45 08 48 8b 55 00 <48> 3b 28 0f 85 f9 28 35 fe 48 3b 6a 08 0f 85 ef 28 35 fe 48 89 42 [ 27.052377] RSP: e02b:ffffc9004016fdd0 EFLAGS: 00010246 [ 27.052381] RAX: 6b6b6b6b6b6b6b6b RBX: ffff88800d66e400 RCX: 0000000000000000 [ 27.052385] RDX: 6b6b6b6b6b6b6b6b RSI: 0000000000000000 RDI: 0000000000000000 [ 27.052389] RBP: ffff88800a887040 R08: 0000000000000000 R09: 0000000000000000 [ 27.052393] R10: 0000000000000000 R11: 0000000000000000 R12: ffff888009e46b68 [ 27.052397] R13: 0000000000000200 R14: 0000000000000000 R15: ffff88800a887040 [ 27.052404] FS: 0000000000000000(0000) GS:ffff88808ca57000(0000) knlGS:0000000000000000 [ 27.052408] CS: e030 DS: 0000 ES: 0000 CR0: 0000000080050033 [ 27.052412] CR2: 00007f9714004360 CR3: 0000000004834000 CR4: 0000000000050660 [ 27.052418] Call Trace: [ 27.052420] <TASK> [ 27.052422] xen_9pfs_front_changed+0x5d5/0x720 [ 27.052426] ? xenbus_otherend_changed+0x72/0x140 [ 27.052430] ? __pfx_xenwatch_thread+0x10/0x10 [ 27.052434] xenwatch_thread+0x94/0x1c0 [ 27.052438] ? __pfx_autoremove_wake_function+0x10/0x10 [ 27.052442] kthread+0xf8/0x240 [ 27.052445] ? __pfx_kthread+0x10/0x10 [ 27.052449] ? __pfx_kthread+0x10/0x10 [ 27.052452] ret_from_fork+0x16b/0x1a0 [ 27.052456] ? __pfx_kthread+0x10/0x10 [ 27.052459] ret_from_fork_asm+0x1a/0x30 [ 27.052463] </TASK> [ 27.052465] Modules linked in: [ 27.052471] ---[ end trace 0000000000000000 ]---

Affected

• Linux >= c15fe55d14b3b4ded5af2a3260877460a6ffb8ad < 480aa6ffd4ac7036eea042047d0cca71bc2074bb
• Linux >= c15fe55d14b3b4ded5af2a3260877460a6ffb8ad < a5d00dff97118a32fcf5fec7a4c3f864c4620c4e
• Linux >= c15fe55d14b3b4ded5af2a3260877460a6ffb8ad < 59e7707492576bdbfa8c1dbe7d90791df31e4773
• Linux >= c15fe55d14b3b4ded5af2a3260877460a6ffb8ad < bf841d43f7a33d75675ba7f4e214ac1c67913065
• Linux >= c15fe55d14b3b4ded5af2a3260877460a6ffb8ad < ce8ded2e61f47747e31eeefb44dc24a2160a7e32
• Linux be03c4fe72384366fd4077a70966bd3b8fc49013
• Linux 1ab4de11232e83b875b071aa44d1155634ca8a1e
• Linux 7cc9dbae8a5f73bd555130384ea256018d28f283
• Linux 3e0359f151ac151abe3fa71040e450ed69cb824b
• Linux 8d3fc907d060c4fb33203e616a395a22083b6566
• Linux 4f0e9244770f5b75a16d8c0929063cd336926764
• Linux 5f6a8974e9ef317fe63f88bab1f33070195dd147
• Linux >= 4.14.308 < 4.15
• Linux >= 4.19.276 < 4.20
• Linux >= 5.4.235 < 5.5
• Linux >= 5.10.173 < 5.11
• Linux >= 5.15.100 < 5.16
• Linux >= 6.1.18 < 6.2
• Linux >= 6.2.5 < 6.3
• Linux 6.3

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.