Bugzilla – Bug 1224851
VUL-0: CVE-2021-47226: kernel: x86/fpu: Invalidate FPU state after a failed XRSTOR from a user buffer
Last modified: 2024-07-19 11:31:59 UTC
In the Linux kernel, the following vulnerability has been resolved: x86/fpu: Invalidate FPU state after a failed XRSTOR from a user buffer Both Intel and AMD consider it to be architecturally valid for XRSTOR to fail with #PF but nonetheless change the register state. The actual conditions under which this might occur are unclear [1], but it seems plausible that this might be triggered if one sibling thread unmaps a page and invalidates the shared TLB while another sibling thread is executing XRSTOR on the page in question. __fpu__restore_sig() can execute XRSTOR while the hardware registers are preserved on behalf of a different victim task (using the fpu_fpregs_owner_ctx mechanism), and, in theory, XRSTOR could fail but modify the registers. If this happens, then there is a window in which __fpu__restore_sig() could schedule out and the victim task could schedule back in without reloading its own FPU registers. This would result in part of the FPU state that __fpu__restore_sig() was attempting to load leaking into the victim task's user-visible state. Invalidate preserved FPU registers on XRSTOR failure to prevent this situation from corrupting any state. [1] Frequent readers of the errata lists might imagine "complex microarchitectural conditions". References: http://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2021-47226 https://www.cve.org/CVERecord?id=CVE-2021-47226 https://git.kernel.org/stable/c/002665dcba4bbec8c82f0aeb4bd3f44334ed2c14 https://git.kernel.org/stable/c/a7748e021b9fb7739e3cb88449296539de0b6817 https://git.kernel.org/stable/c/d8778e393afa421f1f117471144f8ce6deb6953a https://git.kernel.org/pub/scm/linux/security/vulns.git/plain/cve/published/2021/CVE-2021-47226.mbox
cve/linux-5.3-LTSS performs __fpu_invalidate_fpregs_state(fpu) unconditionally before the load. And makes the fpu state valid (by fpregs_mark_activate()) only if the load succeeded. So d8778e393afa would be a nop in case of cve/linux-5.3-LTSS.