[PATCH 5.11 333/601] x86/kprobes: Fix to check non boostable prefixes correctly

Greg Kroah-Hartman
2021-05-12T17:39:26Z

From: Masami Hiramatsu <mhiramat@kernel.org>

[ Upstream commit 6dd3b8c9f58816a1354be39559f630cd1bd12159 ]

There are 2 bugs in the can_boost() function because of using
x86 insn decoder. Since the insn->opcode never has a prefix byte,
it can not find CS override prefix in it. And the insn->attr is
the attribute of the opcode, thus inat_is_address_size_prefix(
insn->attr) always returns false.

Fix those by checking each prefix bytes with for_each_insn_prefix
loop and getting the correct attribute for each prefix byte.
Also, this removes unlikely, because this is a slow path.

Fixes: a8d11cd0714f ("kprobes/x86: Consolidate insn decoder users for copying code")
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/161666691162.1120877.2808435205294352583.stgit@devnote2
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
arch/x86/kernel/kprobes/core.c | 17 ++++++++++++-----
1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
index a65e9e97857f..4e81d86a1470 100644
--- a/arch/x86/kernel/kprobes/core.c
+++ b/arch/x86/kernel/kprobes/core.c
@@ -159,6 +159,8 @@ NOKPROBE_SYMBOL(skip_prefixes);
int can_boost(struct insn *insn, void *addr)
{
kprobe_opcode_t opcode;
+ insn_byte_t prefix;
+ int i;

if (search_exception_tables((unsigned long)addr))
return 0; /* Page fault may occur on this address. */
@@ -171,9 +173,14 @@ int can_boost(struct insn *insn, void *addr)
if (insn->opcode.nbytes != 1)
return 0;

- /* Can't boost Address-size override prefix */
- if (unlikely(inat_is_address_size_prefix(insn->attr)))
- return 0;
+ for_each_insn_prefix(insn, i, prefix) {
+ insn_attr_t attr;
+
+ attr = inat_get_opcode_attribute(prefix);
+ /* Can't boost Address-size override prefix and CS override prefix */
+ if (prefix == 0x2e || inat_is_address_size_prefix(attr))
+ return 0;
+ }

opcode = insn->opcode.bytes[0];

@@ -198,8 +205,8 @@ int can_boost(struct insn *insn, void *addr)
/* clear and set flags are boostable */
return (opcode == 0xf5 || (0xf7 < opcode && opcode < 0xfe));
default:
- /* CS override prefix and call are not boostable */
- return (opcode != 0x2e && opcode != 0x9a);
+ /* call is not boostable */
+ return opcode != 0x9a;
}
}

--
2.30.2