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kcsan: Avoid checking scoped accesses from nested contexts

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Avoid checking scoped accesses from nested contexts (such as nested
interrupts or in scheduler code) which share the same kcsan_ctx.

This is to avoid detecting false positive races of accesses in the same
thread with currently scoped accesses: consider setting up a watchpoint
for a non-scoped (normal) access that also "conflicts" with a current
scoped access. In a nested interrupt (or in the scheduler), which shares
the same kcsan_ctx, we cannot check scoped accesses set up in the parent
context -- simply ignore them in this case.

With the introduction of kcsan_ctx::disable_scoped, we can also clean up
kcsan_check_scoped_accesses()'s recursion guard, and do not need to
modify the list's prev pointer.

Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
This commit is contained in:
Marco Elver 2021-11-30 12:44:11 +01:00 • committed by Paul E. McKenney
parent 71f8de7092
commit 9756f64c8f
2 changed files with 16 additions and 3 deletions
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1
include/linux/kcsan.h
View file

@ -21,6 +21,7 @@
*/
struct kcsan_ctx {
int disable_count; /* disable counter */
int disable_scoped; /* disable scoped access counter */
int atomic_next; /* number of following atomic ops */
/*

18
kernel/kcsan/core.c
View file

@ -204,15 +204,17 @@ check_access(const volatile void *ptr, size_t size, int type, unsigned long ip);
static noinline void kcsan_check_scoped_accesses(void)
{
struct kcsan_ctx *ctx = get_ctx();
struct list_head *prev_save = ctx->scoped_accesses.prev;
struct kcsan_scoped_access *scoped_access;
ctx->scoped_accesses.prev = NULL; /* Avoid recursion. */
if (ctx->disable_scoped)
return;
ctx->disable_scoped++;
list_for_each_entry(scoped_access, &ctx->scoped_accesses, list) {
check_access(scoped_access->ptr, scoped_access->size,
scoped_access->type, scoped_access->ip);
}
ctx->scoped_accesses.prev = prev_save;
ctx->disable_scoped--;
}
/* Rules for generic atomic accesses. Called from fast-path. */
@ -465,6 +467,15 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type, unsigned
goto out;
}
/*
* Avoid races of scoped accesses from nested interrupts (or scheduler).
* Assume setting up a watchpoint for a non-scoped (normal) access that
* also conflicts with a current scoped access. In a nested interrupt,
* which shares the context, it would check a conflicting scoped access.
* To avoid, disable scoped access checking.
*/
ctx->disable_scoped++;
/*
* Save and restore the IRQ state trace touched by KCSAN, since KCSAN's
* runtime is entered for every memory access, and potentially useful
@ -578,6 +589,7 @@ out_unlock:
if (!kcsan_interrupt_watcher)
local_irq_restore(irq_flags);
kcsan_restore_irqtrace(current);
ctx->disable_scoped--;
out:
user_access_restore(ua_flags);
}