From: Peter Zijlstra peterz@infradead.org
commit e52fc2cf3f662828cc0d51c4b73bed73ad275fce upstream.
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 [cascardo: remove ENDBR] Signed-off-by: Thadeu Lima de Souza Cascardo cascardo@canonical.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- arch/x86/kernel/ftrace.c | 9 ++------- arch/x86/kernel/ftrace_64.S | 19 +++++++++++++++---- 2 files changed, 17 insertions(+), 11 deletions(-)
--- a/arch/x86/kernel/ftrace.c +++ b/arch/x86/kernel/ftrace.c @@ -322,12 +322,12 @@ create_trampoline(struct ftrace_ops *ops 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;
@@ -365,12 +365,7 @@ create_trampoline(struct ftrace_ops *ops 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) { --- a/arch/x86/kernel/ftrace_64.S +++ b/arch/x86/kernel/ftrace_64.S @@ -181,7 +181,6 @@ SYM_INNER_LABEL(ftrace_graph_call, SYM_L
/* * 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 @@ -335,7 +334,7 @@ SYM_FUNC_START(ftrace_graph_caller) SYM_FUNC_END(ftrace_graph_caller)
SYM_FUNC_START(return_to_handler) - subq $24, %rsp + subq $16, %rsp
/* Save the return values */ movq %rax, (%rsp) @@ -347,7 +346,19 @@ 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