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

diff --git a/arch/x86/mm/mem_encrypt_identity.c b/arch/x86/mm/mem_encrypt_identity.c
index d73aeb16417fcfbb99607c3bc2241962e3590948..7f72472a34d6d9ac141bb0eb4e7f98de9397698f 100644 (file)
--- 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;