Bug 2536758 (CVE-2026-93167)

Summary: CVE-2026-93167 kernel: csky: Fix a4/a5 restoration in syscall trace path
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
Status: NEW --- QA Contact:
Severity: medium Docs Contact:
Priority: medium    
Version: unspecifiedCC: akhatavk, aos-team-art-private, asdas, dpaolell, jdelft, jupierce, lgarciaa, mbiarnes, ppalepu, ppostler, prdhamdh, rhel-process-autobot, sghai, sidsharm, suppawar, vlaad, watson-tool-maintainers
Target Milestone: ---Keywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in the Linux kernel. When tracing system calls on C-SKY ABIv2, an issue in restoring register values (a4/a5) can lead to incorrect arguments being passed to certain system calls. This can cause traced applications to fail or behave unexpectedly, potentially resulting in a denial of service for the affected process.
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Description OSIDB Bzimport 2026-09-17 19:26:37 UTC 
In the Linux kernel, the following vulnerability has been resolved:

csky: Fix a4/a5 restoration in syscall trace path

The syscall trace path reloads syscall arguments from pt_regs before
calling the syscall handler. On C-SKY ABIv2, the 5th and 6th syscall
arguments are prepared as stack arguments before invoking syscallid.

The current code adjusts sp before loading LSAVE_A4 and LSAVE_A5. Since
those offsets are relative to the original pt_regs base, loading them
after changing sp fetches the wrong slots. As a result, traced syscalls
that use the 5th or 6th argument may receive corrupted arguments.

This is visible with mmap2(), which takes six arguments. A small
PTRACE_SYSCALL reproducer opens a file and maps one page with:

  mmap(NULL, 4096, PROT_READ | PROT_EXEC, MAP_PRIVATE, fd, 0)

Before the fix, the traced child fails the mmap and exits with 12.
After the fix, the mapping succeeds and the child exits with 0.

Fix the trace path by loading a4/a5 from pt_regs before changing sp.

Tested on: ck860f, linux-4.19.15, C-SKY abiv2
Comment 1 gkamathe 2026-09-18 04:53:04 UTC 
Upstream advisory:
https://git.kernel.org/pub/scm/linux/security/vulns.git/plain/cve/published/2026/CVE-2026-93167.mbox