Subject: CVE-2024-35802: x86/sev: Fix position dependent variable references in startup code

From: Greg Kroah-Hartman <[email-masked]>

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 "rip_rel_ref" everywhere like other code shortens
    "rIP-relative reference" and make the asm wrapper __always_inline. ]

The Linux kernel CVE team has assigned CVE-2024-35802 to this issue.

Affected and fixed versions
===========================

	Fixed in 6.1.84 with commit fe272b61506b
	Fixed in 6.6.24 with commit 0982fd6bf0b8
	Fixed in 6.7.12 with commit 66fa3fcb474b
	Fixed in 6.8.3 with commit 954a4a878144
	Fixed in 6.9 with commit 1c811d403afd

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2024-35802
will be updated if fixes are backported, please check that for the most
up to date information about this issue.

Affected files
==============

The file(s) affected by this issue are:
	arch/x86/coco/core.c
	arch/x86/include/asm/asm.h
	arch/x86/include/asm/coco.h
	arch/x86/include/asm/mem_encrypt.h
	arch/x86/kernel/sev-shared.c
	arch/x86/kernel/sev.c
	arch/x86/mm/mem_encrypt_identity.c

Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	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