[PATCH] kcsan: Avoid division by zero in kcsan_prandom_u32_max()

[Andrea Parri <parri.andrea@gmail.com>]

KCSAN's skip_watch, udelay_task and udelay_interrupt parameters can be
changed at runtime through /sys/module/kcsan/parameters/, where 0 means
watching every access or not delaying at all. With the default
CONFIG_KCSAN_SKIP_WATCH_RANDOMIZE=y and CONFIG_KCSAN_DELAY_RANDOMIZE=y,
writing 0 to any of them makes the next watchpoint setup divide by zero:

  # echo 0 > /sys/module/kcsan/parameters/udelay_task
  Oops: divide error: 0000 [#1] SMP NOPTI
  CPU: 0 UID: 0 PID: 93 Comm: sh Not tainted 7.3.0-rc5 #3 PREEMPT(lazy)
  RIP: 0010:kcsan_setup_watchpoint+0x1b5/0x410
  Code: ... 65 89 05 ad 8d 98 02 d3 ee <f7> f6 29 d7 e8 22 0e 0f 01 ...
  RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
  ...
   __show_trace_log_lvl+0x1bd/0x315
   __die_body.cold+0x19/0x2b
   die+0x34/0x50
  ...
  BUG: TASK stack guard page was hit at ffffd3ab001fbfe8 (...)

reset_kcsan_skip() and delay_access() pass these values unchecked to
kcsan_prandom_u32_max(), which returns "state % ep_ro". delay_access()
halves the delay for compound and assert accesses, so
udelay_interrupt=1 is enough there.

The oops path runs instrumented code, which faults again until the stack
overflows. With skip_watch=0 the guest hangs without printing anything.

The division came in with commit 71a076f4a61a ("kcsan: Rewrite
kcsan_prandom_u32_max() without prandom_u32_state()"). Before it, the
helper scaled the random value with a multiply-shift, which yields 0 for
ep_ro == 0. Go back to that via reciprocal_scale().

Tested in an x86-64 KCSAN guest with virtme-ng: after this patch,
writing 0 to each parameter, or 1 to udelay_interrupt, no longer
oopses. skip_watch=0 now watches every access, which is very slow with
the default delays but completes when they are set to 0.

Fixes: 71a076f4a61a ("kcsan: Rewrite kcsan_prandom_u32_max() without prandom_u32_state()")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Andrea Parri <parri.andrea@gmail.com>
---
 kernel/kcsan/core.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/kernel/kcsan/core.c b/kernel/kcsan/core.c
index 2db82661cd60a..a3aa5e5b039dc 100644
--- a/kernel/kcsan/core.c
+++ b/kernel/kcsan/core.c
@@ -290,8 +290,9 @@ should_watch(struct kcsan_ctx *ctx, const volatile void *ptr, size_t size, int t
 }
 
 /*
- * Returns a pseudo-random number in interval [0, ep_ro). Simple linear
- * congruential generator, using constants from "Numerical Recipes".
+ * Returns a pseudo-random number in interval [0, ep_ro), or 0 if ep_ro is 0,
+ * which the runtime-tunable callers may pass. Simple linear congruential
+ * generator, using constants from "Numerical Recipes".
  */
 static u32 kcsan_prandom_u32_max(u32 ep_ro)
 {
@@ -300,7 +301,7 @@ static u32 kcsan_prandom_u32_max(u32 ep_ro)
 	state = 1664525 * state + 1013904223;
 	this_cpu_write(kcsan_rand_state, state);
 
-	return state % ep_ro;
+	return reciprocal_scale(state, ep_ro);
 }
 
 static inline void reset_kcsan_skip(void)
-- 
2.53.0