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x86/alternative: Fix race in try_get_desc()

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commit efd608fa74 upstream.

I encountered some occasional crashes of poke_int3_handler() when
kprobes are set, while accessing desc->vec.

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. ]

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 as a
global.  Anyhow, there is only a single descriptor at any given
moment.

Fixes: 1f676247f3 ("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
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Nadav Amit 2022-09-21 18:09:32 +00:00 • committed by Justin Hammond
parent 568a96cb5e
commit 3acb47bc9d
1 changed files with 23 additions and 22 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

45
arch/x86/kernel/alternative.c
View file

@ -1200,22 +1200,23 @@ 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);
}
@ -1248,15 +1249,15 @@ noinstr int poke_int3_handler(struct pt_regs *regs)
/*
* 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;
@ -1310,7 +1311,7 @@ noinstr int poke_int3_handler(struct pt_regs *regs)
ret = 1;
out_put:
put_desc(desc);
put_desc();
return ret;
}
@ -1341,18 +1342,20 @@ 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
@ -1440,12 +1443,10 @@ static void text_poke_bp_batch(struct text_poke_loc *tp, unsigned int nr_entries
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,