[PATCH] x86/mce: Check regs before accessing it

[Borislav Petkov <bp@alien8.de>]

From: Borislav Petkov <bp@suse.de>

Commit in Fixes accesses pt_regs before checking whether it is NULL or
not. Change the flow so that the NULL pointer check happens first.

Fixes: 0a5b288e85bb ("x86/mce: Prevent severity computation from being instrumented")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Link: https://lore.kernel.org/r/20211217102029.GA29708@kili
---
 arch/x86/kernel/cpu/mce/severity.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/mce/severity.c b/arch/x86/kernel/cpu/mce/severity.c
index a32646769705..7aa2bda93cbb 100644
--- a/arch/x86/kernel/cpu/mce/severity.c
+++ b/arch/x86/kernel/cpu/mce/severity.c
@@ -222,6 +222,9 @@ static bool is_copy_from_user(struct pt_regs *regs)
 	struct insn insn;
 	int ret;
 
+	if (!regs)
+		return false;
+
 	if (copy_from_kernel_nofault(insn_buf, (void *)regs->ip, MAX_INSN_SIZE))
 		return false;
 
@@ -283,7 +286,7 @@ static noinstr int error_context(struct mce *m, struct pt_regs *regs)
 	switch (fixup_type) {
 	case EX_TYPE_UACCESS:
 	case EX_TYPE_COPY:
-		if (!regs || !copy_user)
+		if (!copy_user)
 			return IN_KERNEL;
 		m->kflags |= MCE_IN_KERNEL_COPYIN;
 		fallthrough;
-- 
2.29.2


-- 
Regards/Gruss,
    Boris.

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