diff mbox series

[v2,2/2] x86/kprobes: Prohibit probing on compiler generated CFI checking code

Message ID 168904025785.116016.12766408611437534723.stgit@devnote2
State New
Headers
Series x86: kprobes: Fix CFI_CLANG related issues |

Commit Message

Masami Hiramatsu (Google) July 11, 2023, 1:50 a.m. UTC 
  From: Masami Hiramatsu (Google) <mhiramat@kernel.org>

Prohibit probing on the compiler generated CFI typeid checking code
because it is used for decoding typeid when CFI error happens.

The compiler generates the following instruction sequence for indirect
call checks on x86;

   movl    -<id>, %r10d       ; 6 bytes
   addl    -4(%reg), %r10d    ; 4 bytes
   je      .Ltmp1             ; 2 bytes
   ud2                        ; <- regs->ip

And handle_cfi_failure() decodes these instructions (movl and addl)
for the typeid and the target address. Thus if we put a kprobe on
those instructions, the decode will fail and report a wrong typeid
and target address.


Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 arch/x86/kernel/kprobes/core.c |   34 ++++++++++++++++++++++++++++++++++
 1 file changed, 34 insertions(+)

Comments

Masami Hiramatsu (Google) July 26, 2023, 3:23 a.m. UTC | #1 
On Tue, 11 Jul 2023 10:50:58 +0900
"Masami Hiramatsu (Google)" <mhiramat@kernel.org> wrote:

> From: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> 
> Prohibit probing on the compiler generated CFI typeid checking code
> because it is used for decoding typeid when CFI error happens.
> 
> The compiler generates the following instruction sequence for indirect
> call checks on x86;
> 
>    movl    -<id>, %r10d       ; 6 bytes
>    addl    -4(%reg), %r10d    ; 4 bytes
>    je      .Ltmp1             ; 2 bytes
>    ud2                        ; <- regs->ip
> 
> And handle_cfi_failure() decodes these instructions (movl and addl)
> for the typeid and the target address. Thus if we put a kprobe on
> those instructions, the decode will fail and report a wrong typeid
> and target address.
> 
> 

Hi Peter,

Can I pick this to probes/fixes branch ?

Thank you,

> Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
>  arch/x86/kernel/kprobes/core.c |   34 ++++++++++++++++++++++++++++++++++
>  1 file changed, 34 insertions(+)
> 
> diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
> index f7f6042eb7e6..fa8c2b41cbaf 100644
> --- a/arch/x86/kernel/kprobes/core.c
> +++ b/arch/x86/kernel/kprobes/core.c
> @@ -54,6 +54,7 @@
>  #include <asm/insn.h>
>  #include <asm/debugreg.h>
>  #include <asm/ibt.h>
> +#include <asm/cfi.h>
>  
>  #include "common.h"
>  
> @@ -293,7 +294,40 @@ static int can_probe(unsigned long paddr)
>  #endif
>  		addr += insn.length;
>  	}
> +	if (IS_ENABLED(CONFIG_CFI_CLANG)) {
> +		/*
> +		 * The compiler generates the following instruction sequence
> +		 * for indirect call checks and cfi.c decodes this;
> +		 *
> +		 *   movl    -<id>, %r10d       ; 6 bytes
> +		 *   addl    -4(%reg), %r10d    ; 4 bytes
> +		 *   je      .Ltmp1             ; 2 bytes
> +		 *   ud2                        ; <- regs->ip
> +		 *   .Ltmp1:
> +		 *
> +		 * Also, these movl and addl are used for showing expected
> +		 * type. So those must not be touched.
> +		 */
> +		__addr = recover_probed_instruction(buf, addr);
> +		if (!__addr)
> +			return 0;
> +
> +		if (insn_decode_kernel(&insn, (void *)__addr) < 0)
> +			return 0;
> +
> +		if (insn.opcode.value == 0xBA)
> +			offset = 12;
> +		else if (insn.opcode.value == 0x3)
> +			offset = 6;
> +		else
> +			goto out;
> +
> +		/* This movl/addl is used for decoding CFI. */
> +		if (is_cfi_trap(addr + offset))
> +			return 0;
> +	}
>  
> +out:
>  	return (addr == paddr);
>  }
>  
>
Peter Zijlstra July 26, 2023, 9:29 a.m. UTC | #2 
On Wed, Jul 26, 2023 at 12:23:17PM +0900, Masami Hiramatsu wrote:
> On Tue, 11 Jul 2023 10:50:58 +0900
> "Masami Hiramatsu (Google)" <mhiramat@kernel.org> wrote:
> 
> > From: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> > 
> > Prohibit probing on the compiler generated CFI typeid checking code
> > because it is used for decoding typeid when CFI error happens.
> > 
> > The compiler generates the following instruction sequence for indirect
> > call checks on x86;
> > 
> >    movl    -<id>, %r10d       ; 6 bytes
> >    addl    -4(%reg), %r10d    ; 4 bytes
> >    je      .Ltmp1             ; 2 bytes
> >    ud2                        ; <- regs->ip
> > 
> > And handle_cfi_failure() decodes these instructions (movl and addl)
> > for the typeid and the target address. Thus if we put a kprobe on
> > those instructions, the decode will fail and report a wrong typeid
> > and target address.
> > 
> > 
> 
> Hi Peter,
> 
> Can I pick this to probes/fixes branch ?

I'll stick them in tip/x86/core, that ok?
Masami Hiramatsu (Google) July 27, 2023, 12:40 a.m. UTC | #3 
On Wed, 26 Jul 2023 11:29:17 +0200
Peter Zijlstra <peterz@infradead.org> wrote:

> On Wed, Jul 26, 2023 at 12:23:17PM +0900, Masami Hiramatsu wrote:
> > On Tue, 11 Jul 2023 10:50:58 +0900
> > "Masami Hiramatsu (Google)" <mhiramat@kernel.org> wrote:
> > 
> > > From: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> > > 
> > > Prohibit probing on the compiler generated CFI typeid checking code
> > > because it is used for decoding typeid when CFI error happens.
> > > 
> > > The compiler generates the following instruction sequence for indirect
> > > call checks on x86;
> > > 
> > >    movl    -<id>, %r10d       ; 6 bytes
> > >    addl    -4(%reg), %r10d    ; 4 bytes
> > >    je      .Ltmp1             ; 2 bytes
> > >    ud2                        ; <- regs->ip
> > > 
> > > And handle_cfi_failure() decodes these instructions (movl and addl)
> > > for the typeid and the target address. Thus if we put a kprobe on
> > > those instructions, the decode will fail and report a wrong typeid
> > > and target address.
> > > 
> > > 
> > 
> > Hi Peter,
> > 
> > Can I pick this to probes/fixes branch ?
> 
> I'll stick them in tip/x86/core, that ok?

Yes, since it is for CFI change. 

Thank you,
diff mbox series

Patch

diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
index f7f6042eb7e6..fa8c2b41cbaf 100644
--- a/arch/x86/kernel/kprobes/core.c
+++ b/arch/x86/kernel/kprobes/core.c
@@ -54,6 +54,7 @@ 
 #include <asm/insn.h>
 #include <asm/debugreg.h>
 #include <asm/ibt.h>
+#include <asm/cfi.h>
 
 #include "common.h"
 
@@ -293,7 +294,40 @@  static int can_probe(unsigned long paddr)
 #endif
 		addr += insn.length;
 	}
+	if (IS_ENABLED(CONFIG_CFI_CLANG)) {
+		/*
+		 * The compiler generates the following instruction sequence
+		 * for indirect call checks and cfi.c decodes this;
+		 *
+		 *   movl    -<id>, %r10d       ; 6 bytes
+		 *   addl    -4(%reg), %r10d    ; 4 bytes
+		 *   je      .Ltmp1             ; 2 bytes
+		 *   ud2                        ; <- regs->ip
+		 *   .Ltmp1:
+		 *
+		 * Also, these movl and addl are used for showing expected
+		 * type. So those must not be touched.
+		 */
+		__addr = recover_probed_instruction(buf, addr);
+		if (!__addr)
+			return 0;
+
+		if (insn_decode_kernel(&insn, (void *)__addr) < 0)
+			return 0;
+
+		if (insn.opcode.value == 0xBA)
+			offset = 12;
+		else if (insn.opcode.value == 0x3)
+			offset = 6;
+		else
+			goto out;
+
+		/* This movl/addl is used for decoding CFI. */
+		if (is_cfi_trap(addr + offset))
+			return 0;
+	}
 
+out:
 	return (addr == paddr);
 }