Re: [bug report] x86/mce: Prevent severity computation from being instrumented

[Borislav Petkov <bp@alien8.de>]

+ Tony.

On Fri, Dec 17, 2021 at 01:20:29PM +0300, Dan Carpenter wrote:
> Hello Borislav Petkov,
> 
> This is a semi-automatic email about new static checker warnings.
> 
> The patch 0a5b288e85bb: "x86/mce: Prevent severity computation from
> being instrumented" from Oct 13, 2021, leads to the following Smatch
> complaint:
> 
>     arch/x86/kernel/cpu/mce/severity.c:286 error_context()
>     warn: variable dereferenced before check 'regs' (see line 280)
> 
> arch/x86/kernel/cpu/mce/severity.c
>    279		fixup_type = ex_get_fixup_type(m->ip);
>    280		copy_user  = is_copy_from_user(regs);
>                                                ^^^^
> New unchecked dereference
> 
>    281		instrumentation_end();
>    282	
>    283		switch (fixup_type) {
>    284		case EX_TYPE_UACCESS:
>    285		case EX_TYPE_COPY:
>    286			if (!regs || !copy_user)
>                              ^^^^
> Old code checked for NULL
> 
>    287				return IN_KERNEL;
>    288			m->kflags |= MCE_IN_KERNEL_COPYIN;

Good catch, thanks!

I guess we can do something like this:

---
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;

-- 
Regards/Gruss,
    Boris.

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