[PATCH v2] x86/kprobes: Fix 1 byte conditional jump target

[PATCH v2] x86/kprobes: Fix 1 byte conditional jump target

[Nadav Amit]

From: Nadav Amit <namit@vmware.com>

Commit 3bc753c06dd0 ("kbuild: treat char as always unsigned") broke
kprobes.  Setting a probe-point on 1 byte conditional jump can cause the
kernel to crash, as the branch target is not sign-extended and instead
zero-extended.

In fact, there is no need for any casting of immediate.value since sign
extension is already done during its decoding in insn_get_immediate().

Fix by removing the casting of the 1 byte conditional jump target.
Future patches can also remove the casting (and sign extension) in other
cases in which immediate.value is being used.

Fixes: 3bc753c06dd0 ("kbuild: treat char as always unsigned")
Suggested-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Suggested-by: Dave Hansen <dave.hansen@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Nadav Amit <namit@vmware.com>

---

v1->v2: Removing the casting completely [Dave]
---
 arch/x86/kernel/kprobes/core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
index b36f3c367cb2..695873c0f50b 100644
--- a/arch/x86/kernel/kprobes/core.c
+++ b/arch/x86/kernel/kprobes/core.c
@@ -625,7 +625,7 @@ static int prepare_emulation(struct kprobe *p, struct insn *insn)
 		/* 1 byte conditional jump */
 		p->ainsn.emulate_op = kprobe_emulate_jcc;
 		p->ainsn.jcc.type = opcode & 0xf;
-		p->ainsn.rel32 = *(char *)insn->immediate.bytes;
+		p->ainsn.rel32 = insn->immediate.value;
 		break;
 	case 0x0f:
 		opcode = insn->opcode.bytes[1];
-- 
2.34.1

Re: [PATCH v2] x86/kprobes: Fix 1 byte conditional jump target

[Peter Zijlstra]

On Wed, Feb 08, 2023 at 07:17:08AM +0000, Nadav Amit wrote:
> From: Nadav Amit <namit@vmware.com>
> 
> Commit 3bc753c06dd0 ("kbuild: treat char as always unsigned") broke
> kprobes.  Setting a probe-point on 1 byte conditional jump can cause the
> kernel to crash, as the branch target is not sign-extended and instead
> zero-extended.
> 
> In fact, there is no need for any casting of immediate.value since sign
> extension is already done during its decoding in insn_get_immediate().
> 
> Fix by removing the casting of the 1 byte conditional jump target.
> Future patches can also remove the casting (and sign extension) in other
> cases in which immediate.value is being used.
> 
> Fixes: 3bc753c06dd0 ("kbuild: treat char as always unsigned")
> Suggested-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> Suggested-by: Dave Hansen <dave.hansen@intel.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Nadav Amit <namit@vmware.com>

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>

Re: [PATCH v2] x86/kprobes: Fix 1 byte conditional jump target

[Masami Hiramatsu]

On Wed,  8 Feb 2023 07:17:08 +0000
Nadav Amit <nadav.amit@gmail.com> wrote:

> From: Nadav Amit <namit@vmware.com>
> 
> Commit 3bc753c06dd0 ("kbuild: treat char as always unsigned") broke
> kprobes.  Setting a probe-point on 1 byte conditional jump can cause the
> kernel to crash, as the branch target is not sign-extended and instead
> zero-extended.
> 
> In fact, there is no need for any casting of immediate.value since sign
> extension is already done during its decoding in insn_get_immediate().
> 
> Fix by removing the casting of the 1 byte conditional jump target.
> Future patches can also remove the casting (and sign extension) in other
> cases in which immediate.value is being used.
> 
> Fixes: 3bc753c06dd0 ("kbuild: treat char as always unsigned")
> Suggested-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> Suggested-by: Dave Hansen <dave.hansen@intel.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Nadav Amit <namit@vmware.com>

Thanks, looks good to me.

Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>


> 
> ---
> 
> v1->v2: Removing the casting completely [Dave]
> ---
>  arch/x86/kernel/kprobes/core.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
> index b36f3c367cb2..695873c0f50b 100644
> --- a/arch/x86/kernel/kprobes/core.c
> +++ b/arch/x86/kernel/kprobes/core.c
> @@ -625,7 +625,7 @@ static int prepare_emulation(struct kprobe *p, struct insn *insn)
>  		/* 1 byte conditional jump */
>  		p->ainsn.emulate_op = kprobe_emulate_jcc;
>  		p->ainsn.jcc.type = opcode & 0xf;
> -		p->ainsn.rel32 = *(char *)insn->immediate.bytes;
> +		p->ainsn.rel32 = insn->immediate.value;
>  		break;
>  	case 0x0f:
>  		opcode = insn->opcode.bytes[1];
> -- 
> 2.34.1
> 

Nadav
-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>