CVE: CVE-2024-38601
Published: 2024-06-19
Description: In the Linux kernel, the following vulnerability has been resolved:

ring-buffer: Fix a race between readers and resize checks

The reader code in rb_get_reader_page() swaps a new reader page into the
ring buffer by doing cmpxchg on old->list.prev->next to point it to the
new page. Following that, if the operation is successful,
old->list.next->prev gets updated too. This means the underlying
doubly-linked list is temporarily inconsistent, page->prev->next or
page->next->prev might not be equal back to page for some page in the
ring buffer.

The resize operation in ring_buffer_resize() can be invoked in parallel.
It calls rb_check_pages() which can detect the described inconsistency
and stop further tracing:

[  190.271762] ------------[ cut here ]------------
[  190.271771] WARNING: CPU: 1 PID: 6186 at kernel/trace/ring_buffer.c:1467 rb_check_pages.isra.0+0x6a/0xa0
[  190.271789] Modules linked in: [...]
[  190.271991] Unloaded tainted modules: intel_uncore_frequency(E):1 skx_edac(E):1
[  190.272002] CPU: 1 PID: 6186 Comm: cmd.sh Kdump: loaded Tainted: G            E      6.9.0-rc6-default #5 158d3e1e6d0b091c34c3b96bfd99a1c58306d79f
[  190.272011] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.16.0-0-gd239552c-rebuilt.opensuse.org 04/01/2014
[  190.272015] RIP: 0010:rb_check_pages.isra.0+0x6a/0xa0
[  190.272023] Code: [...]
[  190.272028] RSP: 0018:ffff9c37463abb70 EFLAGS: 00010206
[  190.272034] RAX: ffff8eba04b6cb80 RBX: [PII_REDACTED] RCX: ffff8eba01f13d80
[  190.272038] RDX: ffff8eba01f130c0 RSI: ffff8eba04b6cd00 RDI: ffff8eba0004c700
[  190.272042] RBP: ffff8eba0004c700 R08: [PII_REDACTED] R09: [PII_REDACTED]
[  190.272045] R10: 00000000ffff7f52 R11: ffff8eba7f600000 R12: ffff8eba0004c720
[  190.272049] R13: ffff8eba00223a00 R14: [PII_REDACTED] R15: ffff8eba067a8000
[  190.272053] FS:  00007f1bd64752c0(0000) GS:ffff8eba7f680000(0000) knlGS:[PII_REDACTED]
[  190.272057] CS:  0010 DS: 0000 ES: 0000 CR0: [PII_REDACTED]
[  190.272061] CR2: 00007f1bd6662590 CR3:[PII_REDACTED]91e001 CR4:[PII_REDACTED]370ef0
[  190.272070] DR0: [PII_REDACTED] DR1: [PII_REDACTED] DR2: [PII_REDACTED]
[  190.272073] DR3: [PII_REDACTED] DR6: 00000000fffe0ff0 DR7: [PII_REDACTED]
[  190.272077] Call Trace:
[  190.272098]  <TASK>
[  190.272189]  ring_buffer_resize+0x2ab/0x460
[  190.272199]  __tracing_resize_ring_buffer.part.0+0x23/0xa0
[  190.272206]  tracing_resize_ring_buffer+0x65/0x90
[  190.272216]  tracing_entries_write+0x74/0xc0
[  190.272225]  vfs_write+0xf5/0x420
[  190.272248]  ksys_write+0x67/0xe0
[  190.272256]  do_syscall_64+0x82/0x170
[  190.272363]  entry_SYSCALL_64_after_hwframe+0x76/0x7e
[  190.272373] RIP: 0033:0x7f1bd657d263
[  190.272381] Code: [...]
[  190.272385] RSP: 002b:00007ffe72b643f8 EFLAGS: 00000246 ORIG_RAX: [PII_REDACTED]
[  190.272391] RAX: ffffffffffffffda RBX: [PII_REDACTED] RCX: 00007f1bd657d263
[  190.272395] RDX: [PII_REDACTED] RSI: 0000555a6eb538e0 RDI: [PII_REDACTED]
[  190.272398] RBP: 0000555a6eb538e0 R08:[PII_REDACTED]00000a R09: [PII_REDACTED]
[  190.272401] R10: 0000555a6eb55190 R11: [PII_REDACTED] R12: 00007f1bd6662500
[  190.272404] R13: [PII_REDACTED] R14: 00007f1bd6667c00 R15: [PII_REDACTED]
[  190.272412]  </TASK>
[  190.272414] ---[ end trace [PII_REDACTED] ]---

Note that ring_buffer_resize() calls rb_check_pages() only if the parent
trace_buffer has recording disabled. Recent commit d78ab792705c
("tracing: Stop current tracer when resizing buffer") causes that it is
now always the case which makes it more likely to experience this issue.

The window to hit this race is nonetheless very small. To help
reproducing it, one can add a delay loop in rb_get_reader_page():

 ret = rb_head_page_replace(reader, cpu_buffer->reader_page);
 if (!ret)
 	goto spin;
 for (unsigned i = 0; i < 1U << 26; i++)  /* inserted delay loop */
 	__asm__ __volatile__ ("" : : : "memory");
 rb_list_head(reader->list.next)->prev = &cpu_buffer->reader_page->list;

.. 
---truncated---
CVSS Score: 4.7 (MEDIUM)
CWE: CWE-362
References: https://git.kernel.org/stable/c/1e160196042cac946798ac192a0bc3398f1aa66b; https://git.kernel.org/stable/c/54c64967ba5f8658ae7da76005024ebd3d9d8f6e; https://git.kernel.org/stable/c/595363182f28786d641666a09e674b852c83b4bb