Bug 2541196 (CVE-2026-98044)

Summary: CVE-2026-98044 kernel: bpf: Reject legacy packet loads from callbacks
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
Status: NEW --- QA Contact:
Severity: low Docs Contact:
Priority: low    
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
Target Release: ---   
Hardware: All   
OS: Linux   
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A flaw was found in the Linux kernel's BPF (Berkeley Packet Filter) verifier. This issue allows a privileged local user to cause a Denial of Service (DoS) or unexpected program rejection by loading a specially crafted BPF program containing legacy packet-load instructions inside callback functions. Due to an internal logic error during register state analysis on simulated return paths, the verifier fails validation and triggers a kernel warning.
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Description OSIDB Bzimport 2026-09-25 10:58:49 UTC 
In the Linux kernel, the following vulnerability has been resolved:

bpf: Reject legacy packet loads from callbacks

check_ld_abs() models a failed BPF_LD_ABS or BPF_LD_IND in a
subprogram as an implicit return with R0 set to zero. It calls
prepare_func_exit() to explore this synthesized path.

When the load is reached directly from a synchronous callback,
prepare_func_exit() enforces the callback return contract and marks R0
precise. R0 is not derived from a real instruction on this path, so
precision backtracking reaches the callback call with R0 still requested
and triggers the "callback unexpected regs" verifier bug. A privileged
program loader can therefore cause a verifier warning and an -EFAULT
BPF_PROG_LOAD.

These legacy packet-load instructions are deprecated. Reject them from
callbacks rather than complicating their implicit-return model. Check all
active frames before constructing the implicit return so nested static
subprograms cannot hide the callback context.

Global functions are verified independently with a fresh frame zero, so
an active-frame check cannot identify a global function called from a
callback. Also check the complete subprogram call graph during stack-depth
validation and reject a function containing a legacy load when any caller
is a callback. This covers global and static descendants without making
has_ld_abs transitive, preserving its per-function BTF return-type check.
Ordinary uses outside callbacks remain supported.
Comment 1 Keith Grant 2026-09-28 23:08:31 UTC 
Upstream advisory:
https://git.kernel.org/pub/scm/linux/security/vulns.git/plain/cve/published/2026/CVE-2026-98044.mbox