From f2cb4f95b7571f2bebcf226cd92b448fd58950ca Mon Sep 17 00:00:00 2001 From: Peter Zijlstra Date: Mon, 11 Nov 2019 14:02:10 +0100 Subject: [PATCH] x86/kprobe: Add comments to arch_{,un}optimize_kprobes() Add a few words describing how it is safe to overwrite the 4 bytes after a kprobe. In specific it is possible the JMP.d32 required for the optimized kprobe overwrites multiple instructions. Tested-by: Alexei Starovoitov Tested-by: Steven Rostedt (VMware) Signed-off-by: Peter Zijlstra (Intel) Acked-by: Alexei Starovoitov Cc: Andy Lutomirski Cc: Borislav Petkov Cc: Brian Gerst Cc: Denys Vlasenko Cc: H. Peter Anvin Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Thomas Gleixner Link: https://lkml.kernel.org/r/20191111132458.401696663@infradead.org Signed-off-by: Ingo Molnar --- arch/x86/kernel/kprobes/opt.c | 14 ++++++++++++-- 1 file changed, 12 insertions(+), 2 deletions(-) diff --git a/arch/x86/kernel/kprobes/opt.c b/arch/x86/kernel/kprobes/opt.c index 26e0d6c4d8d6..3f45b5c43a71 100644 --- a/arch/x86/kernel/kprobes/opt.c +++ b/arch/x86/kernel/kprobes/opt.c @@ -414,8 +414,12 @@ int arch_prepare_optimized_kprobe(struct optimized_kprobe *op, } /* - * Replace breakpoints (int3) with relative jumps. + * Replace breakpoints (INT3) with relative jumps (JMP.d32). * Caller must call with locking kprobe_mutex and text_mutex. + * + * The caller will have installed a regular kprobe and after that issued + * syncrhonize_rcu_tasks(), this ensures that the instruction(s) that live in + * the 4 bytes after the INT3 are unused and can now be overwritten. */ void arch_optimize_kprobes(struct list_head *oplist) { @@ -441,7 +445,13 @@ void arch_optimize_kprobes(struct list_head *oplist) } } -/* Replace a relative jump with a breakpoint (int3). */ +/* + * Replace a relative jump (JMP.d32) with a breakpoint (INT3). + * + * After that, we can restore the 4 bytes after the INT3 to undo what + * arch_optimize_kprobes() scribbled. This is safe since those bytes will be + * unused once the INT3 lands. + */ void arch_unoptimize_kprobe(struct optimized_kprobe *op) { arch_arm_kprobe(&op->kp);