Messages in this thread Patch in this message | | | Date | Wed, 11 Dec 2019 01:44:01 +0900 | From | Masami Hiramatsu <> | Subject | Re: [PATCH -tip 1/2] x86/alternative: Sync bp_patching update for avoiding NULL pointer exception |
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Hi Peter,
On Mon, 9 Dec 2019 15:39:40 +0100 Peter Zijlstra <peterz@infradead.org> wrote:
> 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.
Ah, this reminds me of my atomic-refcounter method for kpatch idea and module unloading.
This looks good, but I feel it is a bit complicated.
If we use atomic (and spin-wait) here, can we use atomic_inc_not_zero() in the poke_int3_handler() at first for making sure the bp_batching is under operation or not? I think it makes things simpler, like below.
--------- atomic_t bp_refcnt;
poke_int3_handler() { smp_rmb(); if (!READ_ONCE(bp_patching.nr_entries)) return 0; if (!atomic_inc_not_zero(&bp_refcnt)) return 0; smp_mb__after_atomic(); [use bp_patching] atomic_dec(&bp_refcnt); }
text_poke_bp_batch() { bp_patching.vec = tp; bp_patching.nr_entries = nr_entries; smp_wmb(); atomic_inc(&bp_refcnt); ... atomic_dec(&bp_refcnt); /* wait for all running poke_int3_handler(). */ atomic_cond_read_acquire(&bp_refcnt, !VAL); bp_patching.vec = NULL; bp_patching.nr_entries = 0; } ---------
Thank you,
> > --- > 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; > } >
-- Masami Hiramatsu <mhiramat@kernel.org>
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