[PATCH v2] powerpc/rtas_flash: fix a potential buffer overflow

[Yi Zhuang]

Since snprintf() returns the possible output size instead of the
actual output size, the available flash_msg length returned by
get_validate_flash_msg may exceed the given buffer limit when
simple_read_from_buffer calls copy_to_user

Reported-by: kernel test robot <lkp@xxxxxxxxx>
Fixes: a94a14720eaf5 powerpc/rtas_flash: Fix validate_flash buffer overflow issue
Signed-off-by: Yi Zhuang <zhuangyi1@xxxxxxxxxx>
---
 arch/powerpc/kernel/rtas_flash.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/kernel/rtas_flash.c b/arch/powerpc/kernel/rtas_flash.c
index a99179d83538..062f0724c2ff 100644
--- a/arch/powerpc/kernel/rtas_flash.c
+++ b/arch/powerpc/kernel/rtas_flash.c
@@ -470,9 +470,14 @@ static int get_validate_flash_msg(struct rtas_validate_flash_t *args_buf,
 	if (args_buf->status >= VALIDATE_TMP_UPDATE) { 
 		n = sprintf(msg, "%d\n", args_buf->update_results);
 		if ((args_buf->update_results >= VALIDATE_CUR_UNKNOWN) ||
-		    (args_buf->update_results == VALIDATE_TMP_UPDATE))
+		    (args_buf->update_results == VALIDATE_TMP_UPDATE)) {
 			n += snprintf(msg + n, msglen - n, "%s\n",
 					args_buf->buf);
+			if (n >= msglen) {
+				n = msglen;
+				printk(KERN_ERR "FLASH: msg too long.\n");
+			}
+		}
 	} else {
 		n = sprintf(msg, "%d\n", args_buf->status);
 	}
-- 
2.26.0.106.g9fadedd