[4.19] x86/purgatory: Do not use __builtin_memcpy and __builtin_memset

Message ID 20190813211930.42094-1-ndesaulniers@google.com
State New
Headers show
Series
  • [4.19] x86/purgatory: Do not use __builtin_memcpy and __builtin_memset
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Commit Message

Nick Desaulniers Aug. 13, 2019, 9:19 p.m.
commit 4ce97317f41d38584fb93578e922fcd19e535f5b upstream.

Implementing memcpy and memset in terms of __builtin_memcpy and
__builtin_memset is problematic.

GCC at -O2 will replace calls to the builtins with calls to memcpy and
memset (but will generate an inline implementation at -Os).  Clang will
replace the builtins with these calls regardless of optimization level.
$ llvm-objdump -dr arch/x86/purgatory/string.o | tail

0000000000000339 memcpy:
     339: 48 b8 00 00 00 00 00 00 00 00 movabsq $0, %rax
                000000000000033b:  R_X86_64_64  memcpy
     343: ff e0                         jmpq    *%rax

0000000000000345 memset:
     345: 48 b8 00 00 00 00 00 00 00 00 movabsq $0, %rax
                0000000000000347:  R_X86_64_64  memset
     34f: ff e0

Such code results in infinite recursion at runtime. This is observed
when doing kexec.

Instead, reuse an implementation from arch/x86/boot/compressed/string.c.
This requires to implement a stub function for warn(). Also, Clang may
lower memcmp's that compare against 0 to bcmp's, so add a small definition,
too. See also: commit 5f074f3e192f ("lib/string.c: implement a basic bcmp")

Fixes: 8fc5b4d4121c ("purgatory: core purgatory functionality")
Reported-by: Vaibhav Rustagi <vaibhavrustagi@google.com>
Debugged-by: Vaibhav Rustagi <vaibhavrustagi@google.com>
Debugged-by: Manoj Gupta <manojgupta@google.com>
Suggested-by: Alistair Delva <adelva@google.com>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

Tested-by: Vaibhav Rustagi <vaibhavrustagi@google.com>

Cc: stable@vger.kernel.org
Link: https://bugs.chromium.org/p/chromium/issues/detail?id=984056
Link: https://lkml.kernel.org/r/20190807221539.94583-1-ndesaulniers@google.com
---
This failed to cherry-pick back cleanly due to the SPDX license
identifier not existing in arch/x86/purgatory/string.c in 4.19. `git rm`
it anyway.


 arch/x86/boot/string.c         |  8 ++++++++
 arch/x86/purgatory/Makefile    |  3 +++
 arch/x86/purgatory/purgatory.c |  6 ++++++
 arch/x86/purgatory/string.c    | 25 -------------------------
 4 files changed, 17 insertions(+), 25 deletions(-)
 delete mode 100644 arch/x86/purgatory/string.c

-- 
2.23.0.rc1.153.gdeed80330f-goog

Comments

Greg Kroah-Hartman Aug. 14, 2019, 8:04 a.m. | #1
On Tue, Aug 13, 2019 at 02:19:30PM -0700, Nick Desaulniers wrote:
> commit 4ce97317f41d38584fb93578e922fcd19e535f5b upstream.

> 

> Implementing memcpy and memset in terms of __builtin_memcpy and

> __builtin_memset is problematic.

> 

> GCC at -O2 will replace calls to the builtins with calls to memcpy and

> memset (but will generate an inline implementation at -Os).  Clang will

> replace the builtins with these calls regardless of optimization level.

> $ llvm-objdump -dr arch/x86/purgatory/string.o | tail

> 

> 0000000000000339 memcpy:

>      339: 48 b8 00 00 00 00 00 00 00 00 movabsq $0, %rax

>                 000000000000033b:  R_X86_64_64  memcpy

>      343: ff e0                         jmpq    *%rax

> 

> 0000000000000345 memset:

>      345: 48 b8 00 00 00 00 00 00 00 00 movabsq $0, %rax

>                 0000000000000347:  R_X86_64_64  memset

>      34f: ff e0

> 

> Such code results in infinite recursion at runtime. This is observed

> when doing kexec.

> 

> Instead, reuse an implementation from arch/x86/boot/compressed/string.c.

> This requires to implement a stub function for warn(). Also, Clang may

> lower memcmp's that compare against 0 to bcmp's, so add a small definition,

> too. See also: commit 5f074f3e192f ("lib/string.c: implement a basic bcmp")

> 

> Fixes: 8fc5b4d4121c ("purgatory: core purgatory functionality")

> Reported-by: Vaibhav Rustagi <vaibhavrustagi@google.com>

> Debugged-by: Vaibhav Rustagi <vaibhavrustagi@google.com>

> Debugged-by: Manoj Gupta <manojgupta@google.com>

> Suggested-by: Alistair Delva <adelva@google.com>

> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>

> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

> Tested-by: Vaibhav Rustagi <vaibhavrustagi@google.com>

> Cc: stable@vger.kernel.org

> Link: https://bugs.chromium.org/p/chromium/issues/detail?id=984056

> Link: https://lkml.kernel.org/r/20190807221539.94583-1-ndesaulniers@google.com

> ---

> This failed to cherry-pick back cleanly due to the SPDX license

> identifier not existing in arch/x86/purgatory/string.c in 4.19. `git rm`

> it anyway.


Now queued up, thanks.

So the Fixes: tag does not mean this should be backported to anything
older?  It implies this bug has been in the kernel since the 3.17
release.

thanks,

greg k-h
Nick Desaulniers Aug. 14, 2019, 8:41 p.m. | #2
On Wed, Aug 14, 2019 at 1:04 AM Greg KH <gregkh@linuxfoundation.org> wrote:
>

> On Tue, Aug 13, 2019 at 02:19:30PM -0700, Nick Desaulniers wrote:

> > Fixes: 8fc5b4d4121c ("purgatory: core purgatory functionality")

> So the Fixes: tag does not mean this should be backported to anything

> older?  It implies this bug has been in the kernel since the 3.17

> release.


Ah yeah sorry, I should have dropped the fixes tag from the backport,
as I only cared about supporting 4.19+.

-- 
Thanks,
~Nick Desaulniers

Patch

diff --git a/arch/x86/boot/string.c b/arch/x86/boot/string.c
index c4428a176973..2622c0742c92 100644
--- a/arch/x86/boot/string.c
+++ b/arch/x86/boot/string.c
@@ -34,6 +34,14 @@  int memcmp(const void *s1, const void *s2, size_t len)
 	return diff;
 }
 
+/*
+ * Clang may lower `memcmp == 0` to `bcmp == 0`.
+ */
+int bcmp(const void *s1, const void *s2, size_t len)
+{
+	return memcmp(s1, s2, len);
+}
+
 int strcmp(const char *str1, const char *str2)
 {
 	const unsigned char *s1 = (const unsigned char *)str1;
diff --git a/arch/x86/purgatory/Makefile b/arch/x86/purgatory/Makefile
index 3cf302b26332..91ef244026d2 100644
--- a/arch/x86/purgatory/Makefile
+++ b/arch/x86/purgatory/Makefile
@@ -6,6 +6,9 @@  purgatory-y := purgatory.o stack.o setup-x86_$(BITS).o sha256.o entry64.o string
 targets += $(purgatory-y)
 PURGATORY_OBJS = $(addprefix $(obj)/,$(purgatory-y))
 
+$(obj)/string.o: $(srctree)/arch/x86/boot/compressed/string.c FORCE
+	$(call if_changed_rule,cc_o_c)
+
 $(obj)/sha256.o: $(srctree)/lib/sha256.c FORCE
 	$(call if_changed_rule,cc_o_c)
 
diff --git a/arch/x86/purgatory/purgatory.c b/arch/x86/purgatory/purgatory.c
index 025c34ac0d84..7971f7a8af59 100644
--- a/arch/x86/purgatory/purgatory.c
+++ b/arch/x86/purgatory/purgatory.c
@@ -70,3 +70,9 @@  void purgatory(void)
 	}
 	copy_backup_region();
 }
+
+/*
+ * Defined in order to reuse memcpy() and memset() from
+ * arch/x86/boot/compressed/string.c
+ */
+void warn(const char *msg) {}
diff --git a/arch/x86/purgatory/string.c b/arch/x86/purgatory/string.c
deleted file mode 100644
index 795ca4f2cb3c..000000000000
--- a/arch/x86/purgatory/string.c
+++ /dev/null
@@ -1,25 +0,0 @@ 
-/*
- * Simple string functions.
- *
- * Copyright (C) 2014 Red Hat Inc.
- *
- * Author:
- *       Vivek Goyal <vgoyal@redhat.com>
- *
- * This source code is licensed under the GNU General Public License,
- * Version 2.  See the file COPYING for more details.
- */
-
-#include <linux/types.h>
-
-#include "../boot/string.c"
-
-void *memcpy(void *dst, const void *src, size_t len)
-{
-	return __builtin_memcpy(dst, src, len);
-}
-
-void *memset(void *dst, int c, size_t len)
-{
-	return __builtin_memset(dst, c, len);
-}