From a5f7a5bb3b7f28ba7e4fa246775b29a0e5537255 Mon Sep 17 00:00:00 2001 From: Jinke Han Date: Tue, 8 Sep 2026 15:37:42 +0800 Subject: x86/kprobes: Fix crash when probing CS CALL instructions When using eBPF to probe CS CALL instructions within a function, a crash can be triggered. The eBPF tool probes offset 257 of the __hrtimer_run_queues() function: <__hrtimer_run_queues+249>: nopl 0x0(%rax,%rax,1) <__hrtimer_run_queues+254>: mov %r14,%rdi <__hrtimer_run_queues+257>: cs call <__x86_indirect_thunk_r12> <__hrtimer_run_queues+263>: mov %eax,%r12d <__hrtimer_run_queues+266>: xchg %ax,%ax <__hrtimer_run_queues+268>: mov %r13,%rdi Which triggers this crash: BUG: unable to handle page fault for address: 00000000000f41c9 #PF: supervisor write access in kernel mode #PF: error_code(0x0002) - not-present page PGD 0 P4D 0 Oops: 0002 [#1] SMP NOPTI CPU: 1 PID: 0 Comm: swapper/1 Kdump: loaded Tainted: P RIP: 0010:__hrtimer_run_queues+0x106/0x230 Note that __hrtimer_run_queues+0x106 is __hrtimer_run_queues+262, which is at the 6th byte of the above CS CALL instruction. Since the CS CALL instruction occupies 6 bytes, the exception occurred in the middle of that call instruction. The root cause is that when using eBPF tools to probe in the middle of a function, a kprobe with INT3 is used as the underlying implementation. During single-step emulation of the original CALL instruction, int3_emulate_call() assumes that the probed CALL instruction is 5 bytes long. However, the actual CS-prefixed CALL instruction occupies 6 bytes, so it constructs an incorrect exception return address. When the CPU returns from the kprobe handler, the next instruction to be executed is at the address of the last byte of that CS CALL instruction. Coincidentally, starting from that address, the CPU fetches and decodes a completely different instruction, which ultimately triggers a kernel crash. Fix the issue by using the actual instruction length obtained from the instruction decoder when constructing the exception return address, rather than relying on the hardcoded CALL_INSN_SIZE macro. [ mingo: Refined the changelog ] Fixes: 6256e668b7af ("x86/kprobes: Use int3 instead of debug trap for single-step") Suggested-by: Masami Hiramatsu (Google) Signed-off-by: Jinke Han Signed-off-by: Ingo Molnar Reviewed-by: Masami Hiramatsu (Google) Acked-by: Yafang Shao Acked-by: Borislav Petkov Cc: Peter Zijlstra Link: https://patch.msgid.link/20260908073742.GA10517@didi-ThinkCentre-M920t-N000 --- Documentation/process/stable-kernel-rules.rst | 234 ++++++++++++++++++++++++++ 1 file changed, 234 insertions(+) create mode 100644 Documentation/process/stable-kernel-rules.rst (limited to 'Documentation/process/stable-kernel-rules.rst') diff --git a/Documentation/process/stable-kernel-rules.rst b/Documentation/process/stable-kernel-rules.rst new file mode 100644 index 000000000..edf90bbe3 --- /dev/null +++ b/Documentation/process/stable-kernel-rules.rst @@ -0,0 +1,234 @@ +.. _stable_kernel_rules: + +Everything you ever wanted to know about Linux -stable releases +=============================================================== + +Rules on what kind of patches are accepted, and which ones are not, into the +"-stable" tree: + +- It or an equivalent fix must already exist in Linux mainline (upstream). +- It must be obviously correct and tested. +- It cannot be bigger than 100 lines, with context. +- It must follow the + :ref:`Documentation/process/submitting-patches.rst ` + rules. +- It must either fix a real bug that bothers people or just add a device ID. + To elaborate on the former: + + - It fixes a problem like an oops, a hang, data corruption, a real security + issue, a hardware quirk, a build error (but not for things marked + CONFIG_BROKEN), or some "oh, that's not good" issue. + - Serious issues as reported by a user of a distribution kernel may also + be considered if they fix a notable performance or interactivity issue. + As these fixes are not as obvious and have a higher risk of a subtle + regression they should only be submitted by a distribution kernel + maintainer and include an addendum linking to a bugzilla entry if it + exists and additional information on the user-visible impact. + - No "This could be a problem..." type of things like a "theoretical race + condition", unless an explanation of how the bug can be exploited is also + provided. + - No "trivial" fixes without benefit for users (spelling changes, whitespace + cleanups, etc). + + +Procedure for submitting patches to the -stable tree +---------------------------------------------------- + +.. note:: + + Security patches should not be handled (solely) by the -stable review + process but should follow the procedures in + :ref:`Documentation/process/security-bugs.rst `. + +There are three options to submit a change to -stable trees: + +1. Add a 'stable tag' to the description of a patch you then submit for + mainline inclusion. +2. Ask the stable team to pick up a patch already mainlined. +3. Submit a patch to the stable team that is equivalent to a change already + mainlined. + +The sections below describe each of the options in more detail. + +:ref:`option_1` is **strongly** preferred, it is the easiest and most common. +:ref:`option_2` is mainly meant for changes where backporting was not considered +at the time of submission. :ref:`option_3` is an alternative to the two earlier +options for cases where a mainlined patch needs adjustments to apply in older +series (for example due to API changes). + +When using option 2 or 3 you can ask for your change to be included in specific +stable series. When doing so, ensure the fix or an equivalent is applicable, +submitted, or already present in all newer stable trees still supported. This is +meant to prevent regressions that users might later encounter on updating, if +e.g. a fix merged for 5.19-rc1 would be backported to 5.10.y, but not to 5.15.y. + +.. _option_1: + +Option 1 +******** + +To have a patch you submit for mainline inclusion later automatically picked up +for stable trees, add this tag in the sign-off area:: + + Cc: stable@vger.kernel.org + +Use ``Cc: stable@kernel.org`` instead when fixing unpublished vulnerabilities: +it reduces the chance of accidentally exposing the fix to the public by way of +'git send-email', as mails sent to that address are not delivered anywhere. + +Once the patch is mainlined it will be applied to the stable tree without +anything else needing to be done by the author or subsystem maintainer. + +To send additional instructions to the stable team, use a shell-style inline +comment to pass arbitrary or predefined notes: + +* Specify any additional patch prerequisites for cherry picking:: + + Cc: # 3.3.x: a1f84a3: sched: Check for idle + Cc: # 3.3.x: 1b9508f: sched: Rate-limit newidle + Cc: # 3.3.x: fd21073: sched: Fix affinity logic + Cc: # 3.3.x + Signed-off-by: Ingo Molnar + + The tag sequence has the meaning of:: + + git cherry-pick a1f84a3 + git cherry-pick 1b9508f + git cherry-pick fd21073 + git cherry-pick + + Note that for a patch series, you do not have to list as prerequisites the + patches present in the series itself. For example, if you have the following + patch series:: + + patch1 + patch2 + + where patch2 depends on patch1, you do not have to list patch1 as + prerequisite of patch2 if you have already marked patch1 for stable + inclusion. + +* Point out kernel version prerequisites:: + + Cc: # 3.3.x + + The tag has the meaning of:: + + git cherry-pick + + For each "-stable" tree starting with the specified version. + + Note, such tagging is unnecessary if the stable team can derive the + appropriate versions from Fixes: tags. + +* Delay pick up of patches:: + + Cc: # after -rc3 + +* Point out known problems:: + + Cc: # see patch description, needs adjustments for <= 6.3 + +There furthermore is a variant of the stable tag you can use to make the stable +team's backporting tools (e.g AUTOSEL or scripts that look for commits +containing a 'Fixes:' tag) ignore a change:: + + Cc: # reason goes here, and must be present + +.. _option_2: + +Option 2 +******** + +If the patch already has been merged to mainline, send an email to +stable@vger.kernel.org containing the subject of the patch, the commit ID, +why you think it should be applied, and what kernel versions you wish it to +be applied to. + +.. _option_3: + +Option 3 +******** + +Send the patch, after verifying that it follows the above rules, to +stable@vger.kernel.org and mention the kernel versions you wish it to be applied +to. When doing so, you must note the upstream commit ID in the changelog of your +submission with a separate line above the commit text, like this:: + + commit upstream. + +Or alternatively:: + + [ Upstream commit ] + +If the submitted patch deviates from the original upstream patch (for example +because it had to be adjusted for the older API), this must be very clearly +documented and justified in the patch description. + + +Following the submission +------------------------ + +The sender will receive an ACK when the patch has been accepted into the +queue, or a NAK if the patch is rejected. This response might take a few +days, according to the schedules of the stable team members. + +If accepted, the patch will be added to the -stable queue, for review by other +developers and by the relevant subsystem maintainer. + + +Review cycle +------------ + +- When the -stable maintainers decide for a review cycle, the patches will be + sent to the review committee, and the maintainer of the affected area of + the patch (unless the submitter is the maintainer of the area) and CC: to + the linux-kernel mailing list. +- The review committee has 48 hours in which to ACK or NAK the patch. +- If the patch is rejected by a member of the committee, or linux-kernel + members object to the patch, bringing up issues that the maintainers and + members did not realize, the patch will be dropped from the queue. +- The ACKed patches will be posted again as part of release candidate (-rc) + to be tested by developers and testers. +- Usually only one -rc release is made, however if there are any outstanding + issues, some patches may be modified or dropped or additional patches may + be queued. Additional -rc releases are then released and tested until no + issues are found. +- Responding to the -rc releases can be done on the mailing list by sending + a "Tested-by:" email with any testing information desired. The "Tested-by:" + tags will be collected and added to the release commit. +- At the end of the review cycle, the new -stable release will be released + containing all the queued and tested patches. +- Security patches will be accepted into the -stable tree directly from the + security kernel team, and not go through the normal review cycle. + Contact the kernel security team for more details on this procedure. + + +Trees +----- + +- The queues of patches, for both completed versions and in progress + versions can be found at: + + https://git.kernel.org/pub/scm/linux/kernel/git/stable/stable-queue.git + +- The finalized and tagged releases of all stable kernels can be found + in separate branches per version at: + + https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git + +- The release candidate of all stable kernel versions can be found at: + + https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable-rc.git/ + + .. warning:: + The -stable-rc tree is a snapshot in time of the stable-queue tree and + will change frequently, hence will be rebased often. It should only be + used for testing purposes (e.g. to be consumed by CI systems). + + +Review committee +---------------- + +- This is made up of a number of kernel developers who have volunteered for + this task, and a few that haven't. -- cgit v1.3.1