Messages in this thread |  | | Date | Fri, 28 Nov 2025 11:13:28 +0000 | | From | david laight <> | | Subject | Re: [RFC/RFT PATCH 5/6] random: Plug race in preceding patch |
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On Thu, 27 Nov 2025 10:22:32 +0100 Ard Biesheuvel <ardb+git@google.com> wrote:
> From: Ard Biesheuvel <ardb@kernel.org> > > The lockless get_random_uXX() reads the next value from the linear > buffer and then overwrites it with a 0x0 value. This is racy, as the > code might be re-entered by an interrupt handler, and so the store might > redundantly wipe the location accessed by the interrupt context rather > than the interrupted context.
Is overwriting the used value even useful? If someone manages to read the 'last' value, then they can equally read the 'next' one - which is likely to be just as useful. Moreover the zeros tell anyone who has managed the access the buffer which entry will be used next - without having to find 'batch->position'.
There is probably more to gain from putting the control data in a completely different piece of memory from the buffer.
David
> > To plug this race, wipe the preceding location when reading the next > value from the linear buffer. Given that the position is always non-zero > outside of the critical section, this is guaranteed to be safe, and > ensures that the produced values are always wiped from the buffer. > > Signed-off-by: Ard Biesheuvel <ardb@kernel.org> > --- > drivers/char/random.c | 1 + > 1 file changed, 1 insertion(+) > > diff --git a/drivers/char/random.c b/drivers/char/random.c > index 71bd74871540..e8ba460c5c9c 100644 > --- a/drivers/char/random.c > +++ b/drivers/char/random.c > @@ -547,6 +547,7 @@ type get_random_ ##type(void) \ > next = (u64)next_gen << 32; \ > if (likely(batch->position < ARRAY_SIZE(batch->entropy))) { \ > next |= batch->position + 1; /* next-1 is bogus otherwise */ \ > + batch->entropy[batch->position - 1] = 0; \ > ret = batch->entropy[batch->position]; \ > } \ > if (cmpxchg64_local(&batch->posgen, next, next - 1) != next - 1) { \
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