Messages in this thread Patch in this message | | | Date | Mon, 9 Dec 2019 15:39:40 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH -tip 1/2] x86/alternative: Sync bp_patching update for avoiding NULL pointer exception |
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On Wed, Nov 27, 2019 at 02:56:52PM +0900, Masami Hiramatsu wrote:
> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c > index 4552795a8df4..9505096e2cd1 100644 > --- a/arch/x86/kernel/alternative.c > +++ b/arch/x86/kernel/alternative.c > @@ -1134,8 +1134,14 @@ static void text_poke_bp_batch(struct text_poke_loc *tp, unsigned int nr_entries > * sync_core() implies an smp_mb() and orders this store against > * the writing of the new instruction. > */ > - bp_patching.vec = NULL; > bp_patching.nr_entries = 0; > + /* > + * This sync_core () ensures that all int3 handlers in progress > + * have finished. This allows poke_int3_handler () after this to > + * avoid touching bp_paching.vec by checking nr_entries == 0. > + */ > + text_poke_sync(); > + bp_patching.vec = NULL; > }
How's something like this instead? Under the assumption that it is rare to actually hit the INT3 and even more rare to actually hit this race, the below should be a lot cheaper.
--- arch/x86/kernel/alternative.c | 69 +++++++++++++++++++++++++++++++++---------- 1 file changed, 53 insertions(+), 16 deletions(-)
diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c index 30e86730655c..12f2d193109d 100644 --- a/arch/x86/kernel/alternative.c +++ b/arch/x86/kernel/alternative.c @@ -953,6 +953,8 @@ static struct bp_patching_desc { int nr_entries; } bp_patching; +static atomic_t bp_handlers; + static inline void *text_poke_addr(struct text_poke_loc *tp) { return _stext + tp->rel_addr; @@ -973,8 +975,8 @@ NOKPROBE_SYMBOL(patch_cmp); int notrace poke_int3_handler(struct pt_regs *regs) { struct text_poke_loc *tp; + int nr, len, ret = 0; void *ip; - int len; /* * Having observed our INT3 instruction, we now must observe @@ -987,12 +989,21 @@ int notrace poke_int3_handler(struct pt_regs *regs) * Idem for other elements in bp_patching. */ smp_rmb(); - - if (likely(!bp_patching.nr_entries)) + if (!READ_ONCE(bp_patching.nr_entries)) return 0; + atomic_inc(&bp_handlers); + /* + * 'ACQUIRE', everything happens after the increment. + */ + smp_mb__after_atomic(); + + nr = smp_load_acquire(&bp_patching.nr_entries); + if (likely(!nr)) + goto out; + if (user_mode(regs)) - return 0; + goto out; /* * Discount the INT3. See text_poke_bp_batch(). @@ -1002,16 +1013,16 @@ int notrace poke_int3_handler(struct pt_regs *regs) /* * Skip the binary search if there is a single member in the vector. */ - if (unlikely(bp_patching.nr_entries > 1)) { - tp = bsearch(ip, bp_patching.vec, bp_patching.nr_entries, + if (unlikely(nr > 1)) { + tp = bsearch(ip, bp_patching.vec, nr, sizeof(struct text_poke_loc), patch_cmp); if (!tp) - return 0; + goto out; } else { tp = bp_patching.vec; if (text_poke_addr(tp) != ip) - return 0; + goto out; } len = text_opcode_size(tp->opcode); @@ -1023,7 +1034,7 @@ int notrace poke_int3_handler(struct pt_regs *regs) * Someone poked an explicit INT3, they'll want to handle it, * do not consume. */ - return 0; + goto out; case CALL_INSN_OPCODE: int3_emulate_call(regs, (long)ip + tp->rel32); @@ -1038,7 +1049,14 @@ int notrace poke_int3_handler(struct pt_regs *regs) BUG(); } - return 1; + ret = 1; +out: + /* + * 'RELEASE", everything happens before the decrement. + */ + smp_mb__before_atomic(); + atomic_dec(&bp_handlers); + return ret; } NOKPROBE_SYMBOL(poke_int3_handler); @@ -1076,7 +1094,12 @@ static void text_poke_bp_batch(struct text_poke_loc *tp, unsigned int nr_entries lockdep_assert_held(&text_mutex); bp_patching.vec = tp; - bp_patching.nr_entries = nr_entries; + /* + * bp_patching.vec = tp nr = bp_patching.nr_entries + * REL ACQ + * bp_patching.nr_entries = nr_entries tp = bp_patching.vec[] + */ + smp_store_release(&bp_patching.nr_entries, nr_entries); /* * Corresponding read barrier in int3 notifier for making sure the @@ -1134,13 +1157,27 @@ static void text_poke_bp_batch(struct text_poke_loc *tp, unsigned int nr_entries * sync_core() implies an smp_mb() and orders this store against * the writing of the new instruction. */ - bp_patching.nr_entries = 0; + WRITE_ONCE(bp_patching.nr_entries, 0); /* - * This sync_core () call ensures that all INT3 handlers in progress - * have finished. This allows poke_int3_handler() after this to - * avoid touching bp_paching.vec by checking nr_entries == 0. + * nr_entries = 0 bp_handlers++ + * MB MB + * VAL = bp_handlers nr = nr_entries + */ + smp_mb(); + /* + * Guarantee all poke_int3_handler()s that have observed + * @bp_patching.nr_enties have completed before we clear + * bp_patching.vec. + * + * We can't do this before text_poke_sync() because then there + * might still be observable INT3 instructions. + */ + atomic_cond_read_acquire(&bp_handlers, !VAL); + /* + * bp_handlers == 0 tp = bp_patching.vec[] + * ACQ MB + * bp_patching.vec = NULL bp_handlers--; */ - text_poke_sync(); bp_patching.vec = NULL; }
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