[PATCH v3] tpm: vtpm_proxy: Check length to avoid compiler warning

[Kees Cook]

When building with -Warray-bounds under GCC 11.2, this warning was
emitted:

In function 'memset',
    inlined from 'vtpm_proxy_fops_read' at drivers/char/tpm/tpm_vtpm_proxy.c:102:2:
./include/linux/fortify-string.h:43:33: warning: '__builtin_memset' pointer overflow between offset 164 and size [2147483648, 4294967295]
[-Warray-bounds]
   43 | #define __underlying_memset     __builtin_memset
      |                                 ^

This warning appears to be triggered due to the "count < len"
check in vtpm_proxy_fops_read() splitting the CFG[1], and the compiler
attempting to reason about the possible value range in len compared
to the buffer size.

In order to silence this warning, and to keep this code robust if the
use of proxy_dev->req_len ever changes in the future, explicitly check
the size of len before reaching the memset().

[1] https://lore.kernel.org/lkml/CAG48ez1iTF9KegKJrW5a3WzXgCPZJ73nS2_e5esKJRppdzvv8g@xxxxxxxxxxxxxx

Cc: Peter Huewe <peterhuewe@xxxxxx>
Cc: Jarkko Sakkinen <jarkko@xxxxxxxxxx>
Cc: Jason Gunthorpe <jgg@xxxxxxxx>
Cc: linux-integrity@xxxxxxxxxxxxxxx
Reviewed-by: Stefan Berger <stefanb@xxxxxxxxxxxxx>
Link: https://lore.kernel.org/lkml/4b59d305-6858-1514-751a-37853ad777be@xxxxxxxxxxxxx
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
v1: https://lore.kernel.org/lkml/20220113002727.3709495-1-keescook@xxxxxxxxxxxx
v2: https://lore.kernel.org/lkml/20220118183650.3386989-1-keescook@xxxxxxxxxxxx
v3: Add more details to the commit log, including a link to Jann's analysis
---
 drivers/char/tpm/tpm_vtpm_proxy.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/char/tpm/tpm_vtpm_proxy.c b/drivers/char/tpm/tpm_vtpm_proxy.c
index 91c772e38bb5..5c865987ba5c 100644
--- a/drivers/char/tpm/tpm_vtpm_proxy.c
+++ b/drivers/char/tpm/tpm_vtpm_proxy.c
@@ -91,7 +91,7 @@ static ssize_t vtpm_proxy_fops_read(struct file *filp, char __user *buf,
 
 	len = proxy_dev->req_len;
 
-	if (count < len) {
+	if (count < len || len > sizeof(proxy_dev->buffer)) {
 		mutex_unlock(&proxy_dev->buf_lock);
 		pr_debug("Invalid size in recv: count=%zd, req_len=%zd\n",
 			 count, len);
-- 
2.30.2