On Tue, Sep 20, 2022 at 10:47:43PM +0000, Nadav Amit wrote:

Fix this issue with small backportable patch. Instead of trying to make RCU-like behavior for bp_desc, just eliminate the unnecessary level of indirection of bp_desc, and hold the whole descriptor on the stack. Anyhow, there is only a single descriptor at any given moment.

Because of text_mutex; indeed. No idea why I put that thing on the stack.

I've done a few minor edits to your patch, but it otherwise looks good to me.

--- Subject: x86/alternative: Fix race in try_get_desc() From: Nadav Amit namit@vmware.com Date: Tue, 20 Sep 2022 22:47:43 +0000

From: Nadav Amit namit@vmware.com

The text poke mechanism claims to have an RCU-like behavior, but it does not appear that there is any quiescent state to ensure that nobody holds reference to desc. As a result, the following race appears to be possible, which can lead to memory corruption.

CPU0 CPU1 ---- ---- text_poke_bp_batch() -> smp_store_release(&bp_desc, &desc)

[ notice that desc is on the stack ]

poke_int3_handler()

[ int3 might be kprobe's so sync events are do not help ]

-> try_get_desc(descp=&bp_desc) desc = __READ_ONCE(bp_desc)

if (!desc) [false, success] WRITE_ONCE(bp_desc, NULL); atomic_dec_and_test(&desc.refs)

[ success, desc space on the stack is being reused and might have non-zero value. ] arch_atomic_inc_not_zero(&desc->refs)

[ might succeed since desc points to stack memory that was freed and might be reused. ]

I encountered some occasional crashes of poke_int3_handler() when kprobes are set, while accessing desc->vec. The analysis has been done offline and I did not corroborate the cause of the crashes. Yet, it seems that this race might be the root cause.

Fix this issue with small backportable patch. Instead of trying to make RCU-like behavior for bp_desc, just eliminate the unnecessary level of indirection of bp_desc, and hold the whole descriptor on the stack. Anyhow, there is only a single descriptor at any given moment.

Fixes: 1f676247f36a4 ("x86/alternatives: Implement a better poke_int3_handler() completion scheme") Signed-off-by: Nadav Amit namit@vmware.com Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Cc: stable@kernel.org Link: https://lkml.kernel.org/r/20220920224743.3089-1-namit@vmware.com --- arch/x86/kernel/alternative.c | 45 +++++++++++++++++++++--------------------- 1 file changed, 23 insertions(+), 22 deletions(-)

--- a/arch/x86/kernel/alternative.c +++ b/arch/x86/kernel/alternative.c @@ -1319,22 +1319,23 @@ struct bp_patching_desc { atomic_t refs; };

-static struct bp_patching_desc *bp_desc; +static struct bp_patching_desc bp_desc;

static __always_inline -struct bp_patching_desc *try_get_desc(struct bp_patching_desc **descp) +struct bp_patching_desc *try_get_desc(void) { - /* rcu_dereference */ - struct bp_patching_desc *desc = __READ_ONCE(*descp); + struct bp_patching_desc *desc = &bp_desc;

- if (!desc || !arch_atomic_inc_not_zero(&desc->refs)) + if (!arch_atomic_inc_not_zero(&desc->refs)) return NULL;

return desc; }

-static __always_inline void put_desc(struct bp_patching_desc *desc) +static __always_inline void put_desc(void) { + struct bp_patching_desc *desc = &bp_desc; + smp_mb__before_atomic(); arch_atomic_dec(&desc->refs); } @@ -1367,15 +1368,15 @@ noinstr int poke_int3_handler(struct pt_

/* * Having observed our INT3 instruction, we now must observe - * bp_desc: + * bp_desc with non-zero refcount: * - * bp_desc = desc INT3 + * bp_desc.refs = 1 INT3 * WMB RMB - * write INT3 if (desc) + * write INT3 if (bp_desc.refs != 0) */ smp_rmb();

- desc = try_get_desc(&bp_desc); + desc = try_get_desc(); if (!desc) return 0;

@@ -1429,7 +1430,7 @@ noinstr int poke_int3_handler(struct pt_ ret = 1;

out_put: - put_desc(desc); + put_desc(); return ret; }

@@ -1460,18 +1461,20 @@ static int tp_vec_nr; */ static void text_poke_bp_batch(struct text_poke_loc *tp, unsigned int nr_entries) { - struct bp_patching_desc desc = { - .vec = tp, - .nr_entries = nr_entries, - .refs = ATOMIC_INIT(1), - }; unsigned char int3 = INT3_INSN_OPCODE; unsigned int i; int do_sync;

lockdep_assert_held(&text_mutex);

- smp_store_release(&bp_desc, &desc); /* rcu_assign_pointer */ + bp_desc.vec = tp; + bp_desc.nr_entries = nr_entries; + + /* + * Corresponds to the implicit memory barrier in try_get_desc() to + * ensure reading a non-zero refcount provides up to date bp_desc data. + */ + atomic_set_release(&bp_desc.refs, 1);

/* * Corresponding read barrier in int3 notifier for making sure the @@ -1559,12 +1562,10 @@ static void text_poke_bp_batch(struct te text_poke_sync();

/* - * Remove and synchronize_rcu(), except we have a very primitive - * refcount based completion. + * Remove and wait for refs to be zero. */ - WRITE_ONCE(bp_desc, NULL); /* RCU_INIT_POINTER */ - if (!atomic_dec_and_test(&desc.refs)) - atomic_cond_read_acquire(&desc.refs, !VAL); + if (!atomic_dec_and_test(&bp_desc.refs)) + atomic_cond_read_acquire(&bp_desc.refs, !VAL); }

static void text_poke_loc_init(struct text_poke_loc *tp, void *addr,