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x86/kexec: Cope with relocate_kernel() not being at the start of the page

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A few places in the kexec control code page make the assumption that the first
instruction of relocate_kernel is at the very start of the page.

To allow for Clang CFI information to be added to relocate_kernel(), as well
as the general principle of removing unwarranted assumptions, fix them to use
the external __relocate_kernel_start symbol that the linker adds. This means
using a separate addq and subq for calculating offsets, as the assembler can
no longer calculate the delta directly for itself and relocations aren't that
versatile. But those values can at least be used relative to a local label to
avoid absolute relocations.

Turn the jump from relocate_kernel() to identity_mapped() into a real indirect
'jmp *%rsi' too, while touching it. There was no real reason for it to be
a push+ret in the first place, and adding Clang CFI info will also give
objtool enough visibility to start complaining 'return with modified stack
frame' about it.

  [ bp: Massage commit message. ]

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20250109140757.2841269-9-dwmw2@infradead.org
This commit is contained in:
David Woodhouse
2025-01-09 14:04:20 +00:00
committed by Borislav Petkov (AMD)
parent dc6ffa6cd5
commit e536057543
1 changed files with 11 additions and 7 deletions
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18
arch/x86/kernel/relocate_kernel_64.S
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@@ -95,11 +95,10 @@ SYM_CODE_START_NOALIGN(relocate_kernel)
lea PAGE_SIZE(%rsi), %rsp
/* jump to identity mapped page */
addq $(identity_mapped - relocate_kernel), %rsi
pushq %rsi
ANNOTATE_UNRET_SAFE
ret
int3
0: addq $identity_mapped - 0b, %rsi
subq $__relocate_kernel_start - 0b, %rsi
ANNOTATE_RETPOLINE_SAFE
jmp *%rsi
SYM_CODE_END(relocate_kernel)
SYM_CODE_START_LOCAL_NOALIGN(identity_mapped)
@@ -219,16 +218,21 @@ SYM_CODE_START_LOCAL_NOALIGN(identity_mapped)
/* get the re-entry point of the peer system */
popq %rbp
leaq relocate_kernel(%rip), %r8
movq kexec_pa_swap_page(%rip), %r10
movq pa_backup_pages_map(%rip), %rdi
movq kexec_pa_table_page(%rip), %rax
movq %rax, %cr3
/* Find start (and end) of this physical mapping of control page */
leaq (%rip), %r8
ANNOTATE_NOENDBR
andq $PAGE_MASK, %r8
lea PAGE_SIZE(%r8), %rsp
movl $1, %r11d /* Ensure preserve_context flag is set */
call swap_pages
movq kexec_va_control_page(%rip), %rax
addq $(virtual_mapped - relocate_kernel), %rax
0: addq $virtual_mapped - 0b, %rax
subq $__relocate_kernel_start - 0b, %rax
pushq %rax
ANNOTATE_UNRET_SAFE
ret