alloc_gcs() returns an error-encoded pointer on failure, which comes
from do_mmap(), not NULL.
The current NULL check fails to detect errors, which could lead to using
an invalid GCS address.
Use IS_ERR_VALUE() to properly detect errors, consistent with the
check in gcs_alloc_thread_stack().
Fixes: b57180c75c ("arm64/gcs: Implement shadow stack prctl() interface")
Reviewed-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Will Deacon <will@kernel.org>
53c998527f