CVE-2024-35802 Information

Description

In the Linux kernel the following vulnerability has been resolved:

x86/sev: Fix position dependent variable references in startup code

The early startup code executes from a 1:1 mapping of memory which differs from the mapping that the code was linked and/or relocated to run at. The latter mapping is not active yet at this point and so symbol references that rely on it will fault.

Given that the core kernel is built without -fPIC symbol references are typically emitted as absolute and so any such references occuring in the early startup code will therefore crash the kernel.

While an attempt was made to work around this for the early SEV/SME startup code by forcing RIP-relative addressing for certain global SEV/SME variables via inline assembly (see snp_cpuid_get_table() for example) RIP-relative addressing must be pervasively enforced for SEV/SME global variables when accessed prior to page table fixups.

__startup_64() already handles this issue for select non-SEV/SME global variables using fixup_pointer() which adjusts the pointer relative to a physaddr argument. To avoid having to pass around this physaddr argument across all functions needing to apply pointer fixups introduce a macro RIP_RELATIVE_REF() which generates a RIP-relative reference to a given global variable. It is used where necessary to force RIP-relative accesses to global variables.

For backporting purposes this patch makes no attempt at cleaning up other occurrences of this pattern involving either inline asm or fixup_pointer(). Those will be addressed later.

[ bp: Call it ip_rel_ref\ everywhere like other code shortens IP-relative reference\ and make the asm wrapper __always_inline. ]

Reference

https://git.kernel.org/stable/c/fe272b61506bb1534922ef07aa165fd3c37a6a90 https://git.kernel.org/stable/c/0982fd6bf0b822876f2e93ec782c4c28a3f85535 https://git.kernel.org/stable/c/66fa3fcb474b2b892fe42d455a6f7ec5aaa98fb9 https://git.kernel.org/stable/c/954a4a87814465ad61cc97c1cd3de1525baaaf07 https://git.kernel.org/stable/c/1c811d403afd73f04bde82b83b24c754011bd0e8