[PATCH] tpm: vtpm_proxy: Avoid device-originated buffer overflow

[Kees Cook]

When building with -Warray-bounds, 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
      |                                 ^

There was no checking of the req_len value from the device. A malicious
(or buggy) device could end up leaking (and when wiping) memory contents
beyond the end of the proxy buffer.

Cc: Peter Huewe <peterhuewe@xxxxxx>
Cc: Jarkko Sakkinen <jarkko@xxxxxxxxxx>
Cc: Jason Gunthorpe <jgg@xxxxxxxx>
Cc: linux-integrity@xxxxxxxxxxxxxxx
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 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