[PATCH v2] x86/mm: Fix use of uninitialized buffer in sme_enable()

[Nikita Zhandarovich]

cmdline_find_option() may fail before doing any initialization of
buffer array. This may lead to unpredictable results when the same
buffer is used later in calls to strncmp() function.
Fix the issue by returning early if cmdline_find_option() returns -1.

Found by Linux Verification Center (linuxtesting.org) with static analysis
tool SVACE.

Fixes: aca20d546214 ("x86/mm: Add support to make use of Secure Memory Encryption")
Signed-off-by: Nikita Zhandarovich <n.zhandarovich@xxxxxxxxxx>
---
v2: per Borislav Petkov's <bp@xxxxxxxxx> remarks:
- return early if cmdline_find_options() fails with -1 instead of zeroing out
buffer;
- use correct Fixes: commit hash

 arch/x86/mm/mem_encrypt_identity.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/mm/mem_encrypt_identity.c b/arch/x86/mm/mem_encrypt_identity.c
index 88cccd65029d..c6efcf559d88 100644
--- a/arch/x86/mm/mem_encrypt_identity.c
+++ b/arch/x86/mm/mem_encrypt_identity.c
@@ -600,7 +600,8 @@ void __init sme_enable(struct boot_params *bp)
 	cmdline_ptr = (const char *)((u64)bp->hdr.cmd_line_ptr |
 				     ((u64)bp->ext_cmd_line_ptr << 32));
 
-	cmdline_find_option(cmdline_ptr, cmdline_arg, buffer, sizeof(buffer));
+	if (cmdline_find_option(cmdline_ptr, cmdline_arg, buffer, sizeof(buffer)) < 0)
+		return;
 
 	if (!strncmp(buffer, cmdline_on, sizeof(buffer)))
 		sme_me_mask = me_mask;