From: Masami Hiramatsu (Google) <mhiramat@kernel.org>

Since the CONFIG_RETHUNK and CONFIG_SLS will use INT3 for stopping
speculative execution after function return, kprobe jump optimization
always fails on the functions with such INT3 inside the function body.
(It already checks the INT3 padding between functions, but not inside
 the function)

To avoid this issue, as same as kprobes, check whether the INT3 comes
from kgdb or not, and if so, stop decoding and make it fail. The other
INT3 will come from CONFIG_RETHUNK/CONFIG_SLS and those can be
treated as a one-byte instruction.

Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Fixes: e463a09af2f0 ("x86: Add straight-line-speculation mitigation")
Cc: stable@vger.kernel.org
---
 arch/x86/kernel/kprobes/opt.c |   28 ++++++++--------------------
 1 file changed, 8 insertions(+), 20 deletions(-)

diff --git a/arch/x86/kernel/kprobes/opt.c b/arch/x86/kernel/kprobes/opt.c
index e6b8c5362b94..e57e07b0edb6 100644
--- a/arch/x86/kernel/kprobes/opt.c
+++ b/arch/x86/kernel/kprobes/opt.c
@@ -15,6 +15,7 @@
 #include <linux/extable.h>
 #include <linux/kdebug.h>
 #include <linux/kallsyms.h>
+#include <linux/kgdb.h>
 #include <linux/ftrace.h>
 #include <linux/objtool.h>
 #include <linux/pgtable.h>
@@ -279,19 +280,6 @@ static int insn_is_indirect_jump(struct insn *insn)
 	return ret;
 }
 
-static bool is_padding_int3(unsigned long addr, unsigned long eaddr)
-{
-	unsigned char ops;
-
-	for (; addr < eaddr; addr++) {
-		if (get_kernel_nofault(ops, (void *)addr) < 0 ||
-		    ops != INT3_INSN_OPCODE)
-			return false;
-	}
-
-	return true;
-}
-
 /* Decode whole function to ensure any instructions don't jump into target */
 static int can_optimize(unsigned long paddr)
 {
@@ -334,15 +322,15 @@ static int can_optimize(unsigned long paddr)
 		ret = insn_decode_kernel(&insn, (void *)recovered_insn);
 		if (ret < 0)
 			return 0;
-
+#ifdef CONFIG_KGDB
 		/*
-		 * In the case of detecting unknown breakpoint, this could be
-		 * a padding INT3 between functions. Let's check that all the
-		 * rest of the bytes are also INT3.
+		 * If there is a dynamically installed kgdb sw breakpoint,
+		 * this function should not be probed.
 		 */
-		if (insn.opcode.bytes[0] == INT3_INSN_OPCODE)
-			return is_padding_int3(addr, paddr - offset + size) ? 1 : 0;
-
+		if (insn.opcode.bytes[0] == INT3_INSN_OPCODE &&
+		    kgdb_has_hit_break(addr))
+			return 0;
+#endif
 		/* Recover address */
 		insn.kaddr = (void *)addr;
 		insn.next_byte = (void *)(addr + insn.length);

> On Sep 8, 2022, at 8:01 AM, Masami Hiramatsu (Google) <mhiramat@kernel.org> wrote:
> 
> From: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> 
> Since the CONFIG_RETHUNK and CONFIG_SLS will use INT3 for stopping
> speculative execution after function return, kprobe jump optimization
> always fails on the functions with such INT3 inside the function body.
> (It already checks the INT3 padding between functions, but not inside
> the function)
> 
> To avoid this issue, as same as kprobes, check whether the INT3 comes
> from kgdb or not, and if so, stop decoding and make it fail. The other
> INT3 will come from CONFIG_RETHUNK/CONFIG_SLS and those can be
> treated as a one-byte instruction.
> 
> Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> Suggested-by: Peter Zijlstra <peterz@infradead.org>
> Fixes: e463a09af2f0 ("x86: Add straight-line-speculation mitigation")
> Cc: stable@vger.kernel.org
> ---
> arch/x86/kernel/kprobes/opt.c |   28 ++++++++--------------------
> 1 file changed, 8 insertions(+), 20 deletions(-)
> 
> diff --git a/arch/x86/kernel/kprobes/opt.c b/arch/x86/kernel/kprobes/opt.c
> index e6b8c5362b94..e57e07b0edb6 100644
> --- a/arch/x86/kernel/kprobes/opt.c
> +++ b/arch/x86/kernel/kprobes/opt.c
> @@ -15,6 +15,7 @@
> #include <linux/extable.h>
> #include <linux/kdebug.h>
> #include <linux/kallsyms.h>
> +#include <linux/kgdb.h>
> #include <linux/ftrace.h>
> #include <linux/objtool.h>
> #include <linux/pgtable.h>
> @@ -279,19 +280,6 @@ static int insn_is_indirect_jump(struct insn *insn)
> return ret;
> }
> 
> -static bool is_padding_int3(unsigned long addr, unsigned long eaddr)
> -{
> - unsigned char ops;
> -
> - for (; addr < eaddr; addr++) {
> - if (get_kernel_nofault(ops, (void *)addr) < 0 ||
> -    ops != INT3_INSN_OPCODE)
> - return false;
> - }
> -
> - return true;
> -}
> -
> /* Decode whole function to ensure any instructions don't jump into target */
> static int can_optimize(unsigned long paddr)
> {
> @@ -334,15 +322,15 @@ static int can_optimize(unsigned long paddr)
> ret = insn_decode_kernel(&insn, (void *)recovered_insn);
> if (ret < 0)
> return 0;
> -
> +#ifdef CONFIG_KGDB
> /*
> - * In the case of detecting unknown breakpoint, this could be
> - * a padding INT3 between functions. Let's check that all the
> - * rest of the bytes are also INT3.
> + * If there is a dynamically installed kgdb sw breakpoint,
> + * this function should not be probed.
> */
> - if (insn.opcode.bytes[0] == INT3_INSN_OPCODE)
> - return is_padding_int3(addr, paddr - offset + size) ? 1 : 0;
> -
> + if (insn.opcode.bytes[0] == INT3_INSN_OPCODE &&
> +    kgdb_has_hit_break(addr))
> + return 0;
> +#endif
> /* Recover address */
> insn.kaddr = (void *)addr;
> insn.next_byte = (void *)(addr + insn.length);

Hi Masami,

I encountered a similar issue with can_probe(). I see that your
patches were not upstreamed, at least to 6.1.

So I was wondering whether it they are going to be upstreamed, and
whether you want also to update can_probe().

Thanks,
Nadav

On Thu, 15 Dec 2022 03:31:25 +0000
Nadav Amit <namit@vmware.com> wrote:

> 
> > On Sep 8, 2022, at 8:01 AM, Masami Hiramatsu (Google) <mhiramat@kernel.org> wrote:
> > 
> > From: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> > 
> > Since the CONFIG_RETHUNK and CONFIG_SLS will use INT3 for stopping
> > speculative execution after function return, kprobe jump optimization
> > always fails on the functions with such INT3 inside the function body.
> > (It already checks the INT3 padding between functions, but not inside
> > the function)
> > 
> > To avoid this issue, as same as kprobes, check whether the INT3 comes
> > from kgdb or not, and if so, stop decoding and make it fail. The other
> > INT3 will come from CONFIG_RETHUNK/CONFIG_SLS and those can be
> > treated as a one-byte instruction.
> > 
> > Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> > Suggested-by: Peter Zijlstra <peterz@infradead.org>
> > Fixes: e463a09af2f0 ("x86: Add straight-line-speculation mitigation")
> > Cc: stable@vger.kernel.org
> > ---
> > arch/x86/kernel/kprobes/opt.c |   28 ++++++++--------------------
> > 1 file changed, 8 insertions(+), 20 deletions(-)
> > 
> > diff --git a/arch/x86/kernel/kprobes/opt.c b/arch/x86/kernel/kprobes/opt.c
> > index e6b8c5362b94..e57e07b0edb6 100644
> > --- a/arch/x86/kernel/kprobes/opt.c
> > +++ b/arch/x86/kernel/kprobes/opt.c
> > @@ -15,6 +15,7 @@
> > #include <linux/extable.h>
> > #include <linux/kdebug.h>
> > #include <linux/kallsyms.h>
> > +#include <linux/kgdb.h>
> > #include <linux/ftrace.h>
> > #include <linux/objtool.h>
> > #include <linux/pgtable.h>
> > @@ -279,19 +280,6 @@ static int insn_is_indirect_jump(struct insn *insn)
> > return ret;
> > }
> > 
> > -static bool is_padding_int3(unsigned long addr, unsigned long eaddr)
> > -{
> > - unsigned char ops;
> > -
> > - for (; addr < eaddr; addr++) {
> > - if (get_kernel_nofault(ops, (void *)addr) < 0 ||
> > -    ops != INT3_INSN_OPCODE)
> > - return false;
> > - }
> > -
> > - return true;
> > -}
> > -
> > /* Decode whole function to ensure any instructions don't jump into target */
> > static int can_optimize(unsigned long paddr)
> > {
> > @@ -334,15 +322,15 @@ static int can_optimize(unsigned long paddr)
> > ret = insn_decode_kernel(&insn, (void *)recovered_insn);
> > if (ret < 0)
> > return 0;
> > -
> > +#ifdef CONFIG_KGDB
> > /*
> > - * In the case of detecting unknown breakpoint, this could be
> > - * a padding INT3 between functions. Let's check that all the
> > - * rest of the bytes are also INT3.
> > + * If there is a dynamically installed kgdb sw breakpoint,
> > + * this function should not be probed.
> > */
> > - if (insn.opcode.bytes[0] == INT3_INSN_OPCODE)
> > - return is_padding_int3(addr, paddr - offset + size) ? 1 : 0;
> > -
> > + if (insn.opcode.bytes[0] == INT3_INSN_OPCODE &&
> > +    kgdb_has_hit_break(addr))
> > + return 0;
> > +#endif
> > /* Recover address */
> > insn.kaddr = (void *)addr;
> > insn.next_byte = (void *)(addr + insn.length);
> 
> Hi Masami,
> 
> I encountered a similar issue with can_probe(). I see that your
> patches were not upstreamed, at least to 6.1.

Oops, I should update and resend the series. This patch must go
through x86 (tip) tree since it is x86 specific issue. Moreover,
this is a kind of bugfix because kprobes doesn't work correctly
with this issue.

> 
> So I was wondering whether it they are going to be upstreamed, and
> whether you want also to update can_probe().

Yeah, I want to push it.

Thank you!


> 
> Thanks,
> Nadav

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