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Message-ID: <lsq.1520823972.318631229@decadent.org.uk>
Date: Mon, 12 Mar 2018 03:06:12 +0000
From: Ben Hutchings <ben@...adent.org.uk>
To: linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC: akpm@...ux-foundation.org, "Andy Lutomirski" <luto@...capital.net>,
"Denys Vlasenko" <dvlasenk@...hat.com>,
"Oleg Nesterov" <oleg@...hat.com>,
"Borislav Petkov" <bp@...en8.de>, "Ingo Molnar" <mingo@...nel.org>,
"Thomas Gleixner" <tglx@...utronix.de>,
"Quentin Casasnovas" <quentin.casasnovas@...cle.com>,
"Brian Gerst" <brgerst@...il.com>, "Borislav Petkov" <bp@...e.de>,
"Linus Torvalds" <torvalds@...ux-foundation.org>,
"H. Peter Anvin" <hpa@...or.com>
Subject: [PATCH 3.16 14/76] x86/alternatives: Fix ALTERNATIVE_2 padding
generation properly
3.16.56-rc1 review patch. If anyone has any objections, please let me know.
------------------
From: Borislav Petkov <bp@...e.de>
commit dbe4058a6a44af4ca5d146aebe01b0a1f9b7fd2a upstream.
Quentin caught a corner case with the generation of instruction
padding in the ALTERNATIVE_2 macro: if len(orig_insn) <
len(alt1) < len(alt2), then not enough padding gets added and
that is not good(tm) as we could overwrite the beginning of the
next instruction.
Luckily, at the time of this writing, we don't have
ALTERNATIVE_2() invocations which have that problem and even if
we did, a simple fix would be to prepend the instructions with
enough prefixes so that that corner case doesn't happen.
However, best it would be if we fixed it properly. See below for
a simple, abstracted example of what we're doing.
So what we ended up doing is, we compute the
max(len(alt1), len(alt2)) - len(orig_insn)
and feed that value to the .skip gas directive. The max() cannot
have conditionals due to gas limitations, thus the fancy integer
math.
With this patch, all ALTERNATIVE_2 sites get padded correctly;
generating obscure test cases pass too:
#define alt_max_short(a, b) ((a) ^ (((a) ^ (b)) & -(-((a) < (b)))))
#define gen_skip(orig, alt1, alt2, marker) \
.skip -((alt_max_short(alt1, alt2) - (orig)) > 0) * \
(alt_max_short(alt1, alt2) - (orig)),marker
.pushsection .text, "ax"
.globl main
main:
gen_skip(1, 2, 4, 0x09)
gen_skip(4, 1, 2, 0x10)
...
.popsection
Thanks to Quentin for catching it and double-checking the fix!
Reported-by: Quentin Casasnovas <quentin.casasnovas@...cle.com>
Signed-off-by: Borislav Petkov <bp@...e.de>
Cc: Andy Lutomirski <luto@...capital.net>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Brian Gerst <brgerst@...il.com>
Cc: Denys Vlasenko <dvlasenk@...hat.com>
Cc: H. Peter Anvin <hpa@...or.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Oleg Nesterov <oleg@...hat.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Link: http://lkml.kernel.org/r/20150404133443.GE21152@pd.tnic
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
arch/x86/include/asm/alternative-asm.h | 14 ++++++++++++--
arch/x86/include/asm/alternative.h | 16 ++++++++++++----
arch/x86/kernel/alternative.c | 4 ++--
3 files changed, 26 insertions(+), 8 deletions(-)
--- a/arch/x86/include/asm/alternative-asm.h
+++ b/arch/x86/include/asm/alternative-asm.h
@@ -45,12 +45,22 @@
.popsection
.endm
+#define old_len 141b-140b
+#define new_len1 144f-143f
+#define new_len2 145f-144f
+
+/*
+ * max without conditionals. Idea adapted from:
+ * http://graphics.stanford.edu/~seander/bithacks.html#IntegerMinOrMax
+ */
+#define alt_max_short(a, b) ((a) ^ (((a) ^ (b)) & -(-((a) < (b)))))
+
.macro ALTERNATIVE_2 oldinstr, newinstr1, feature1, newinstr2, feature2
140:
\oldinstr
141:
- .skip -(((144f-143f)-(141b-140b)) > 0) * ((144f-143f)-(141b-140b)),0x90
- .skip -(((145f-144f)-(144f-143f)-(141b-140b)) > 0) * ((145f-144f)-(144f-143f)-(141b-140b)),0x90
+ .skip -((alt_max_short(new_len1, new_len2) - (old_len)) > 0) * \
+ (alt_max_short(new_len1, new_len2) - (old_len)),0x90
142:
.pushsection .altinstructions,"a"
--- a/arch/x86/include/asm/alternative.h
+++ b/arch/x86/include/asm/alternative.h
@@ -96,13 +96,21 @@ static inline int alternatives_text_rese
alt_end_marker ":\n"
/*
+ * max without conditionals. Idea adapted from:
+ * http://graphics.stanford.edu/~seander/bithacks.html#IntegerMinOrMax
+ *
+ * The additional "-" is needed because gas works with s32s.
+ */
+#define alt_max_short(a, b) "((" a ") ^ (((" a ") ^ (" b ")) & -(-((" a ") - (" b ")))))"
+
+/*
* Pad the second replacement alternative with additional NOPs if it is
* additionally longer than the first replacement alternative.
*/
-#define OLDINSTR_2(oldinstr, num1, num2) \
- __OLDINSTR(oldinstr, num1) \
- ".skip -(((" alt_rlen(num2) ")-(" alt_rlen(num1) ")-(662b-661b)) > 0) * " \
- "((" alt_rlen(num2) ")-(" alt_rlen(num1) ")-(662b-661b)),0x90\n" \
+#define OLDINSTR_2(oldinstr, num1, num2) \
+ "661:\n\t" oldinstr "\n662:\n" \
+ ".skip -((" alt_max_short(alt_rlen(num1), alt_rlen(num2)) " - (" alt_slen ")) > 0) * " \
+ "(" alt_max_short(alt_rlen(num1), alt_rlen(num2)) " - (" alt_slen ")), 0x90\n" \
alt_end_marker ":\n"
#define ALTINSTR_ENTRY(feature, num) \
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -372,11 +372,11 @@ void __init_or_module apply_alternatives
continue;
}
- DPRINTK("feat: %d*32+%d, old: (%p, len: %d), repl: (%p, len: %d)",
+ DPRINTK("feat: %d*32+%d, old: (%p, len: %d), repl: (%p, len: %d), pad: %d",
a->cpuid >> 5,
a->cpuid & 0x1f,
instr, a->instrlen,
- replacement, a->replacementlen);
+ replacement, a->replacementlen, a->padlen);
DUMP_BYTES(instr, a->instrlen, "%p: old_insn: ", instr);
DUMP_BYTES(replacement, a->replacementlen, "%p: rpl_insn: ", replacement);
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