[tip: x86/sev] x86/sme: Fix memory encryption setting if enabled by default and not overridden

["tip-bot2 for Ard Biesheuvel" <tip-bot2@linutronix.de>]

The following commit has been merged into the x86/sev branch of tip:

Commit-ID:     e814b59e6c2b11f5a3d007b2e61f7d550c354c3a
Gitweb:        https://git.kernel.org/tip/e814b59e6c2b11f5a3d007b2e61f7d550c354c3a
Author:        Ard Biesheuvel <ardb@kernel.org>
AuthorDate:    Fri, 26 Jan 2024 17:39:19 +01:00
Committer:     Borislav Petkov (AMD) <bp@alien8.de>
CommitterDate: Mon, 29 Jan 2024 17:08:33 +01:00

x86/sme: Fix memory encryption setting if enabled by default and not overridden

Commit

  cbebd68f59f0 ("x86/mm: Fix use of uninitialized buffer in sme_enable()")

'fixed' an issue in sme_enable() detected by static analysis, and broke
the common case in the process.

cmdline_find_option() will return < 0 on an error, or when the command
line argument does not appear at all. In this particular case, the
latter is not an error condition, and so the early exit is wrong.

Instead, without mem_encrypt= on the command line, the compile time
default should be honoured, which could be to enable memory encryption,
and this is currently broken.

Fix it by setting sme_me_mask to a preliminary value based on the
compile time default, and only omitting the command line argument test
when cmdline_find_option() returns an error.

  [ bp: Drop active_by_default while at it. ]

Fixes: cbebd68f59f0 ("x86/mm: Fix use of uninitialized buffer in sme_enable()")
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Link: https://lore.kernel.org/r/20240126163918.2908990-2-ardb+git@google.com
---
 arch/x86/mm/mem_encrypt_identity.c | 10 +++-------
 1 file changed, 3 insertions(+), 7 deletions(-)

diff --git a/arch/x86/mm/mem_encrypt_identity.c b/arch/x86/mm/mem_encrypt_identity.c
index d73aeb1..7f72472 100644
--- a/arch/x86/mm/mem_encrypt_identity.c
+++ b/arch/x86/mm/mem_encrypt_identity.c
@@ -507,7 +507,6 @@ void __init sme_enable(struct boot_params *bp)
 	const char *cmdline_ptr, *cmdline_arg, *cmdline_on, *cmdline_off;
 	unsigned int eax, ebx, ecx, edx;
 	unsigned long feature_mask;
-	bool active_by_default;
 	unsigned long me_mask;
 	char buffer[16];
 	bool snp;
@@ -593,22 +592,19 @@ void __init sme_enable(struct boot_params *bp)
 	     : "p" (sme_cmdline_off));
 
 	if (IS_ENABLED(CONFIG_AMD_MEM_ENCRYPT_ACTIVE_BY_DEFAULT))
-		active_by_default = true;
-	else
-		active_by_default = false;
+		sme_me_mask = me_mask;
 
 	cmdline_ptr = (const char *)((u64)bp->hdr.cmd_line_ptr |
 				     ((u64)bp->ext_cmd_line_ptr << 32));
 
 	if (cmdline_find_option(cmdline_ptr, cmdline_arg, buffer, sizeof(buffer)) < 0)
-		return;
+		goto out;
 
 	if (!strncmp(buffer, cmdline_on, sizeof(buffer)))
 		sme_me_mask = me_mask;
 	else if (!strncmp(buffer, cmdline_off, sizeof(buffer)))
 		sme_me_mask = 0;
-	else
-		sme_me_mask = active_by_default ? me_mask : 0;
+
 out:
 	if (sme_me_mask) {
 		physical_mask &= ~sme_me_mask;