[PATCH] lib/crypto: blake2s: avoid indirect calls to compression function for Clang CFI

[John Stultz]

On Fri, Jan 21, 2022 at 11:17 AM Jason A. Donenfeld <Jason at zx2c4.com> wrote:
>
> blake2s_compress_generic is weakly aliased to blake2s_generic. The
> current harness for function selection uses a function pointer, which is
> ordinarily inlined and resolved at compile time. But when Clang's CFI is
> enabled, CFI still triggers when making an indirect call via a weak
> symbol. This seems like a bug in Clang's CFI, as though it's bucketing
> weak symbols and strong symbols differently. It also only seems to
> trigger when "full LTO" mode is used, rather than "thin LTO".
>
> [    0.000000][    T0] Kernel panic - not syncing: CFI failure (target: blake2s_compress_generic+0x0/0x1444)
> [    0.000000][    T0] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.16.0-mainline-06981-g076c855b846e #1
> [    0.000000][    T0] Hardware name: MT6873 (DT)
> [    0.000000][    T0] Call trace:
> [    0.000000][    T0]  dump_backtrace+0xfc/0x1dc
> [    0.000000][    T0]  dump_stack_lvl+0xa8/0x11c
> [    0.000000][    T0]  panic+0x194/0x464
> [    0.000000][    T0]  __cfi_check_fail+0x54/0x58
> [    0.000000][    T0]  __cfi_slowpath_diag+0x354/0x4b0
> [    0.000000][    T0]  blake2s_update+0x14c/0x178
> [    0.000000][    T0]  _extract_entropy+0xf4/0x29c
> [    0.000000][    T0]  crng_initialize_primary+0x24/0x94
> [    0.000000][    T0]  rand_initialize+0x2c/0x6c
> [    0.000000][    T0]  start_kernel+0x2f8/0x65c
> [    0.000000][    T0]  __primary_switched+0xc4/0x7be4
> [    0.000000][    T0] Rebooting in 5 seconds..

YongQin also reported hitting this issue(also, only in the LTO=full
case) on the db845c dev board. Sami pointed me to this patch and I
just wanted to confirm it gets things booting again.

Reported-by: YongQin Liu <yongqin.liu at linaro.org>
Tested-by: John Stultz <john.stultz at linaro.org>

Thanks so much for the quick analysis and fix!
-john