On 2023-08-10 22:45:32, Kees Cook wrote: > If an output buffer size exceeded U16_MAX, the min_t(u16, ...) cast in > copy_data() was causing writes to truncate. This manifested as output > bytes being skipped, seen as %NUL bytes in pstore dumps when the available > record size was larger than 65536. Fix the cast to no longer truncate > the calculation. > > Cc: Petr Mladek <pmladek@xxxxxxxx> > Cc: Sergey Senozhatsky <senozhatsky@xxxxxxxxxxxx> > Cc: Steven Rostedt <rostedt@xxxxxxxxxxx> > Cc: John Ogness <john.ogness@xxxxxxxxxxxxx> > Reported-by: Vijay Balakrishna <vijayb@xxxxxxxxxxxxxxxxxxx> > Closes: https://lore.kernel.org/lkml/d8bb1ec7-a4c5-43a2-9de0-9643a70b899f@xxxxxxxxxxxxxxxxxxx/ > Fixes: b6cf8b3f3312 ("printk: add lockless ringbuffer") > Cc: stable@xxxxxxxxxxxxxxx > Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx> Nice find! Reviewed-by: Tyler Hicks (Microsoft) <code@xxxxxxxxxxx> Tested-by: Tyler Hicks (Microsoft) <code@xxxxxxxxxxx> Verified the fix by applying it to an instrumented v6.5-rc5 kernel that allows userspace to execute kmsg_dump(), detects NULL bytes in data copied from the ring buffer, and warns about invalid truncation due to the min_t(u16, ...) casting bug. Everything looks good! Tyler > --- > kernel/printk/printk_ringbuffer.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/kernel/printk/printk_ringbuffer.c b/kernel/printk/printk_ringbuffer.c > index 2dc4d5a1f1ff..fde338606ce8 100644 > --- a/kernel/printk/printk_ringbuffer.c > +++ b/kernel/printk/printk_ringbuffer.c > @@ -1735,7 +1735,7 @@ static bool copy_data(struct prb_data_ring *data_ring, > if (!buf || !buf_size) > return true; > > - data_size = min_t(u16, buf_size, len); > + data_size = min_t(unsigned int, buf_size, len); > > memcpy(&buf[0], data, data_size); /* LMM(copy_data:A) */ > return true; > -- > 2.34.1 >
