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x86/ibt,ftrace: Make function-graph play nice

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Return trampoline must not use indirect branch to return; while this
preserves the RSB, it is fundamentally incompatible with IBT. Instead
use a retpoline like ROP gadget that defeats IBT while not unbalancing
the RSB.

And since ftrace_stub is no longer a plain RET, don't use it to copy
from. Since RET is a trivial instruction, poke it directly.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lore.kernel.org/r/20220308154318.347296408@infradead.org
This commit is contained in:
Peter Zijlstra 2022-03-08 16:30:31 +01:00
parent d15cb3dab1
commit e52fc2cf3f
2 changed files with 19 additions and 11 deletions
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9
arch/x86/kernel/ftrace.c
View File

@ -316,12 +316,12 @@ create_trampoline(struct ftrace_ops *ops, unsigned int *tramp_size)
unsigned long offset;
unsigned long npages;
unsigned long size;
unsigned long retq;
unsigned long *ptr;
void *trampoline;
void *ip;
/* 48 8b 15 <offset> is movq <offset>(%rip), %rdx */
unsigned const char op_ref[] = { 0x48, 0x8b, 0x15 };
unsigned const char retq[] = { RET_INSN_OPCODE, INT3_INSN_OPCODE };
union ftrace_op_code_union op_ptr;
int ret;
@ -359,12 +359,7 @@ create_trampoline(struct ftrace_ops *ops, unsigned int *tramp_size)
goto fail;
ip = trampoline + size;
/* The trampoline ends with ret(q) */
retq = (unsigned long)ftrace_stub;
ret = copy_from_kernel_nofault(ip, (void *)retq, RET_SIZE);
if (WARN_ON(ret < 0))
goto fail;
memcpy(ip, retq, RET_SIZE);
/* No need to test direct calls on created trampolines */
if (ops->flags & FTRACE_OPS_FL_SAVE_REGS) {

21
arch/x86/kernel/ftrace_64.S
View File

@ -176,10 +176,10 @@ SYM_FUNC_END(ftrace_caller);
SYM_FUNC_START(ftrace_epilogue)
/*
* This is weak to keep gas from relaxing the jumps.
* It is also used to copy the RET for trampolines.
*/
SYM_INNER_LABEL_ALIGN(ftrace_stub, SYM_L_WEAK)
UNWIND_HINT_FUNC
ENDBR
RET
SYM_FUNC_END(ftrace_epilogue)
@ -284,6 +284,7 @@ SYM_FUNC_START(__fentry__)
jnz trace
SYM_INNER_LABEL(ftrace_stub, SYM_L_GLOBAL)
ENDBR
RET
trace:
@ -307,7 +308,7 @@ EXPORT_SYMBOL(__fentry__)
#ifdef CONFIG_FUNCTION_GRAPH_TRACER
SYM_FUNC_START(return_to_handler)
subq $24, %rsp
subq $16, %rsp
/* Save the return values */
movq %rax, (%rsp)
@ -319,7 +320,19 @@ SYM_FUNC_START(return_to_handler)
movq %rax, %rdi
movq 8(%rsp), %rdx
movq (%rsp), %rax
addq $24, %rsp
JMP_NOSPEC rdi
addq $16, %rsp
/*
* Jump back to the old return address. This cannot be JMP_NOSPEC rdi
* since IBT would demand that contain ENDBR, which simply isn't so for
* return addresses. Use a retpoline here to keep the RSB balanced.
*/
ANNOTATE_INTRA_FUNCTION_CALL
call .Ldo_rop
int3
.Ldo_rop:
mov %rdi, (%rsp)
UNWIND_HINT_FUNC
RET
SYM_FUNC_END(return_to_handler)
#endif