Description

In the Linux kernel, the following vulnerability has been resolved: arm64: entry: fix ARM64_WORKAROUND_SPECULATIVE_UNPRIV_LOAD Currently the ARM64_WORKAROUND_SPECULATIVE_UNPRIV_LOAD workaround isn't quite right, as it is supposed to be applied after the last explicit memory access, but is immediately followed by an LDR. The ARM64_WORKAROUND_SPECULATIVE_UNPRIV_LOAD workaround is used to handle Cortex-A520 erratum 2966298 and Cortex-A510 erratum 3117295, which are described in: * https://developer.arm.com/documentation/SDEN2444153/0600/?lang=en * https://developer.arm.com/documentation/SDEN1873361/1600/?lang=en In both cases the workaround is described as: | If pagetable isolation is disabled, the context switch logic in the | kernel can be updated to execute the following sequence on affected | cores before exiting to EL0, and after all explicit memory accesses: | | 1. A non-shareable TLBI to any context and/or address, including | unused contexts or addresses, such as a `TLBI VALE1 Xzr`. | | 2. A DSB NSH to guarantee completion of the TLBI. The important part being that the TLBI+DSB must be placed "after all explicit memory accesses". Unfortunately, as-implemented, the TLBI+DSB is immediately followed by an LDR, as we have: | alternative_if ARM64_WORKAROUND_SPECULATIVE_UNPRIV_LOAD | tlbi vale1, xzr | dsb nsh | alternative_else_nop_endif | alternative_if_not ARM64_UNMAP_KERNEL_AT_EL0 | ldr lr, [sp, #S_LR] | add sp, sp, #PT_REGS_SIZE // restore sp | eret | alternative_else_nop_endif | | [ ... KPTI exception return path ... ] This patch fixes this by reworking the logic to place the TLBI+DSB immediately before the ERET, after all explicit memory accesses. The ERET is currently in a separate alternative block, and alternatives cannot be nested. To account for this, the alternative block for ARM64_UNMAP_KERNEL_AT_EL0 is replaced with a single alternative branch to skip the KPTI logic, with the new shape of the logic being: | alternative_insn "b .L_skip_tramp_exit_\@", nop, ARM64_UNMAP_KERNEL_AT_EL0 | [ ... KPTI exception return path ... ] | .L_skip_tramp_exit_\@: | | ldr lr, [sp, #S_LR] | add sp, sp, #PT_REGS_SIZE // restore sp | | alternative_if ARM64_WORKAROUND_SPECULATIVE_UNPRIV_LOAD | tlbi vale1, xzr | dsb nsh | alternative_else_nop_endif | eret The new structure means that the workaround is only applied when KPTI is not in use; this is fine as noted in the documented implications of the erratum: | Pagetable isolation between EL0 and higher level ELs prevents the | issue from occurring. ... and as per the workaround description quoted above, the workaround is only necessary "If pagetable isolation is disabled".

Metrics

 

CVSS Version 4.0 CVSS Version 3.x CVSS Version 2.0

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CVSS 4.0 Severity and Vector Strings:

NIST: NVD

N/A

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CVSS 3.x Severity and Vector Strings:

NIST: NVD

Base Score:  N/A

NVD assessment not yet provided.

CVSS 2.0 Severity and Vector Strings:

NIST: NVD

Base Score: N/A

NVD assessment not yet provided.

In the Linux kernel, the following vulnerability has been resolved:

arm64: entry: fix ARM64_WORKAROUND_SPECULATIVE_UNPRIV_LOAD

Currently the ARM64_WORKAROUND_SPECULATIVE_UNPRIV_LOAD workaround isn't
quite right, as it is supposed to be applied after the last explicit
memory access, but is immediately followed by an LDR.

The ARM64_WORKAROUND_SPECULATIVE_UNPRIV_LOAD workaround is used to
handle Cortex-A520 erratum 2966298 and Cortex-A510 erratum 3117295,
which are described in:

* https://devel

kernel.org https://git.kernel.org/stable/c/58eb5c07f41704464b9acc09ab0707b6769db6c0 [No types assigned]

kernel.org https://git.kernel.org/stable/c/832dd634bd1b4e3bbe9f10b9c9ba5db6f6f2b97f [No types assigned]

kernel.org https://git.kernel.org/stable/c/baa0aaac16432019651e0d60c41cd34a0c3c3477 [No types assigned]