[PATCH v9 0/6] Improve Minimum Alternate Stack Size

[PATCH v9 0/6] Improve Minimum Alternate Stack Size

[Chang S. Bae]

During signal entry, the kernel pushes data onto the normal userspace
stack. On x86, the data pushed onto the user stack includes XSAVE state,
which has grown over time as new features and larger registers have been
added to the architecture.

MINSIGSTKSZ is a constant provided in the kernel signal.h headers and
typically distributed in lib-dev(el) packages, e.g. [1]. Its value is
compiled into programs and is part of the user/kernel ABI. The MINSIGSTKSZ
constant indicates to userspace how much data the kernel expects to push on
the user stack, [2][3].

However, this constant is much too small and does not reflect recent
additions to the architecture. For instance, when AVX-512 states are in
use, the signal frame size can be 3.5KB while MINSIGSTKSZ remains 2KB.

The bug report [4] explains this as an ABI issue. The small MINSIGSTKSZ can
cause user stack overflow when delivering a signal.

In this series, we suggest a couple of things:
1. Provide a variable minimum stack size to userspace, as a similar
   approach to [5].
2. Avoid using a too-small alternate stack.

Changes from v8 [13]:
* Added and revised some kernel messages. (Borislav Petkov)

Changes from v7 [12]:
* Improved the overflow check code. (Andy Lutomirski and Borislav Petkov)
* Moved the "Fixes:" tag and the bugzilla link (patch 5 -> patch 3).

Changes from v6 [11]:
* Updated and fixed the documentation. (Borislav Petkov)
* Revised the AT_MINSIGSTKSZ comment. (Borislav Petkov)

Changes form v5 [10]:
* Fixed the overflow detection. (Andy Lutomirski)
* Reverted the AT_MINSIGSTKSZ removal on arm64. (Dave Martin)
* Added a documentation about the x86 AT_MINSIGSTKSZ.
* Supported the existing sigaltstack test to use the new aux vector.

Changes from v4 [9]:
* Moved the aux vector define to the generic header. (Carlos O'Donell)

Changes from v3 [8]:
* Updated the changelog. (Borislav Petkov)
* Revised the test messages again. (Borislav Petkov)

Changes from v2 [7]:
* Simplified the sigaltstack overflow prevention. (Jann Horn)
* Renamed fpstate size helper with cleanup. (Borislav Petkov)
* Cleaned up the signframe struct size defines. (Borislav Petkov)
* Revised the selftest messages. (Borislav Petkov)
* Revised a changelog. (Borislav Petkov)

Changes from v1 [6]:
* Took stack alignment into account for sigframe size. (Dave Martin)

[1]: https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/bits/sigstack.h;h=b9dca794da093dc4d41d39db9851d444e1b54d9b;hb=HEAD
[2]: https://www.gnu.org/software/libc/manual/html_node/Signal-Stack.html
[3]: https://man7.org/linux/man-pages/man2/sigaltstack.2.html
[4]: https://bugzilla.kernel.org/show_bug.cgi?id=153531
[5]: https://blog.linuxplumbersconf.org/2017/ocw/system/presentations/4671/original/plumbers-dm-2017.pdf
[6]: https://lore.kernel.org/lkml/20200929205746.6763-1-chang.seok.bae@intel.com/
[7]: https://lore.kernel.org/lkml/20201119190237.626-1-chang.seok.bae@intel.com/
[8]: https://lore.kernel.org/lkml/20201223015312.4882-1-chang.seok.bae@intel.com/
[9]: https://lore.kernel.org/lkml/20210115211038.2072-1-chang.seok.bae@intel.com/
[10]: https://lore.kernel.org/lkml/20210203172242.29644-1-chang.seok.bae@intel.com/
[11]: https://lore.kernel.org/lkml/20210227165911.32757-1-chang.seok.bae@intel.com/
[12]: https://lore.kernel.org/lkml/20210316065215.23768-1-chang.seok.bae@intel.com/
[13]: https://lore.kernel.org/lkml/20210422044856.27250-1-chang.seok.bae@intel.com/

Chang S. Bae (6):
  uapi: Define the aux vector AT_MINSIGSTKSZ
  x86/signal: Introduce helpers to get the maximum signal frame size
  x86/elf: Support a new ELF aux vector AT_MINSIGSTKSZ
  selftest/sigaltstack: Use the AT_MINSIGSTKSZ aux vector if available
  x86/signal: Detect and prevent an alternate signal stack overflow
  selftest/x86/signal: Include test cases for validating sigaltstack

 Documentation/x86/elf_auxvec.rst          |  53 +++++++++
 Documentation/x86/index.rst               |   1 +
 arch/x86/include/asm/elf.h                |   4 +
 arch/x86/include/asm/fpu/signal.h         |   2 +
 arch/x86/include/asm/sigframe.h           |   2 +
 arch/x86/include/uapi/asm/auxvec.h        |   4 +-
 arch/x86/kernel/cpu/common.c              |   3 +
 arch/x86/kernel/fpu/signal.c              |  19 ++++
 arch/x86/kernel/signal.c                  |  88 ++++++++++++++-
 include/linux/sched/signal.h              |  19 ++--
 include/uapi/linux/auxvec.h               |   3 +
 tools/testing/selftests/sigaltstack/sas.c |  20 +++-
 tools/testing/selftests/x86/Makefile      |   2 +-
 tools/testing/selftests/x86/sigaltstack.c | 128 ++++++++++++++++++++++
 14 files changed, 327 insertions(+), 21 deletions(-)
 create mode 100644 Documentation/x86/elf_auxvec.rst
 create mode 100644 tools/testing/selftests/x86/sigaltstack.c


base-commit: d07f6ca923ea0927a1024dfccafc5b53b61cfecc
--
2.17.1

[PATCH v9 1/6] uapi: Define the aux vector AT_MINSIGSTKSZ

[Chang S. Bae]

Define the AT_MINSIGSTKSZ in generic Linux. It is already used as generic
ABI in glibc's generic elf.h, and this define will prevent future namespace
conflicts. In particular, x86 is also using this generic definition.

Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Reviewed-by: Len Brown <len.brown@intel.com>
Cc: Carlos O'Donell <carlos@redhat.com>
Cc: Dave Martin <Dave.Martin@arm.com>
Cc: libc-alpha@sourceware.org
Cc: linux-arch@vger.kernel.org
Cc: linux-api@vger.kernel.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
---
Change from v6:
* Revised the comment. (Borislav Petkov)

Change from v5:
* Reverted the arm64 change. (Dave Martin and Will Deacon)
* Massaged the changelog.

Change from v4:
* Added as a new patch (Carlos O'Donell)
---
 include/uapi/linux/auxvec.h | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/include/uapi/linux/auxvec.h b/include/uapi/linux/auxvec.h
index abe5f2b6581b..c7e502bf5a6f 100644
--- a/include/uapi/linux/auxvec.h
+++ b/include/uapi/linux/auxvec.h
@@ -33,5 +33,8 @@
 
 #define AT_EXECFN  31	/* filename of program */
 
+#ifndef AT_MINSIGSTKSZ
+#define AT_MINSIGSTKSZ	51	/* minimal stack size for signal delivery */
+#endif
 
 #endif /* _UAPI_LINUX_AUXVEC_H */
-- 
2.17.1

[PATCH v9 1/6] uapi: Define the aux vector AT_MINSIGSTKSZ

[Chang S. Bae]

Define the AT_MINSIGSTKSZ in generic Linux. It is already used as generic
ABI in glibc's generic elf.h, and this define will prevent future namespace
conflicts. In particular, x86 is also using this generic definition.

Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Reviewed-by: Len Brown <len.brown@intel.com>
Cc: Carlos O'Donell <carlos@redhat.com>
Cc: Dave Martin <Dave.Martin@arm.com>
Cc: libc-alpha@sourceware.org
Cc: linux-arch@vger.kernel.org
Cc: linux-api@vger.kernel.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
---
Change from v6:
* Revised the comment. (Borislav Petkov)

Change from v5:
* Reverted the arm64 change. (Dave Martin and Will Deacon)
* Massaged the changelog.

Change from v4:
* Added as a new patch (Carlos O'Donell)
---
 include/uapi/linux/auxvec.h | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/include/uapi/linux/auxvec.h b/include/uapi/linux/auxvec.h
index abe5f2b6581b..c7e502bf5a6f 100644
--- a/include/uapi/linux/auxvec.h
+++ b/include/uapi/linux/auxvec.h
@@ -33,5 +33,8 @@
 
 #define AT_EXECFN  31	/* filename of program */
 
+#ifndef AT_MINSIGSTKSZ
+#define AT_MINSIGSTKSZ	51	/* minimal stack size for signal delivery */
+#endif
 
 #endif /* _UAPI_LINUX_AUXVEC_H */
-- 
2.17.1


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http://lists.infradead.org/mailman/listinfo/linux-arm-kernel

[PATCH v9 2/6] x86/signal: Introduce helpers to get the maximum signal frame size

[Chang S. Bae]

Signal frames do not have a fixed format and can vary in size when a number
of things change: support XSAVE features, 32 vs. 64-bit apps. Add the code
to support a runtime method for userspace to dynamically discover how large
a signal stack needs to be.

Introduce a new variable, max_frame_size, and helper functions for the
calculation to be used in a new user interface. Set max_frame_size to a
system-wide worst-case value, instead of storing multiple app-specific
values.

Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Reviewed-by: Len Brown <len.brown@intel.com>
Acked-by: H.J. Lu <hjl.tools@gmail.com>
Cc: x86@kernel.org
Cc: linux-kernel@vger.kernel.org
---
Changes from v8:
* Added a message for the calculated sigframe size. (Borislav Petkov)

Changes from v2:
* Renamed the fpstate size helper with cleanup (Borislav Petkov)
* Moved the sigframe struct size defines to where used (Borislav Petkov)
* Removed unneeded sentence in the changelog (Borislav Petkov)

Change from v1:
* Took stack alignment into account for sigframe size (Dave Martin)
---
 arch/x86/include/asm/fpu/signal.h |  2 ++
 arch/x86/include/asm/sigframe.h   |  2 ++
 arch/x86/kernel/cpu/common.c      |  3 ++
 arch/x86/kernel/fpu/signal.c      | 19 ++++++++++
 arch/x86/kernel/signal.c          | 59 +++++++++++++++++++++++++++++--
 5 files changed, 83 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/fpu/signal.h b/arch/x86/include/asm/fpu/signal.h
index 7fb516b6893a..8b6631dffefd 100644
--- a/arch/x86/include/asm/fpu/signal.h
+++ b/arch/x86/include/asm/fpu/signal.h
@@ -29,6 +29,8 @@ unsigned long
 fpu__alloc_mathframe(unsigned long sp, int ia32_frame,
 		     unsigned long *buf_fx, unsigned long *size);
 
+unsigned long fpu__get_fpstate_size(void);
+
 extern void fpu__init_prepare_fx_sw_frame(void);
 
 #endif /* _ASM_X86_FPU_SIGNAL_H */
diff --git a/arch/x86/include/asm/sigframe.h b/arch/x86/include/asm/sigframe.h
index 84eab2724875..5b1ed650b124 100644
--- a/arch/x86/include/asm/sigframe.h
+++ b/arch/x86/include/asm/sigframe.h
@@ -85,4 +85,6 @@ struct rt_sigframe_x32 {
 
 #endif /* CONFIG_X86_64 */
 
+void __init init_sigframe_size(void);
+
 #endif /* _ASM_X86_SIGFRAME_H */
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index a1b756c49a93..9173dd475818 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -58,6 +58,7 @@
 #include <asm/intel-family.h>
 #include <asm/cpu_device_id.h>
 #include <asm/uv/uv.h>
+#include <asm/sigframe.h>
 
 #include "cpu.h"
 
@@ -1332,6 +1333,8 @@ static void __init early_identify_cpu(struct cpuinfo_x86 *c)
 
 	fpu__init_system(c);
 
+	init_sigframe_size();
+
 #ifdef CONFIG_X86_32
 	/*
 	 * Regardless of whether PCID is enumerated, the SDM says
diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
index a4ec65317a7f..dbb304e48f16 100644
--- a/arch/x86/kernel/fpu/signal.c
+++ b/arch/x86/kernel/fpu/signal.c
@@ -507,6 +507,25 @@ fpu__alloc_mathframe(unsigned long sp, int ia32_frame,
 
 	return sp;
 }
+
+unsigned long fpu__get_fpstate_size(void)
+{
+	unsigned long ret = xstate_sigframe_size();
+
+	/*
+	 * This space is needed on (most) 32-bit kernels, or when a 32-bit
+	 * app is running on a 64-bit kernel. To keep things simple, just
+	 * assume the worst case and always include space for 'freg_state',
+	 * even for 64-bit apps on 64-bit kernels. This wastes a bit of
+	 * space, but keeps the code simple.
+	 */
+	if ((IS_ENABLED(CONFIG_IA32_EMULATION) ||
+	     IS_ENABLED(CONFIG_X86_32)) && use_fxsr())
+		ret += sizeof(struct fregs_state);
+
+	return ret;
+}
+
 /*
  * Prepare the SW reserved portion of the fxsave memory layout, indicating
  * the presence of the extended state information in the memory layout
diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c
index a06cb107c0e8..689a4b6dd18f 100644
--- a/arch/x86/kernel/signal.c
+++ b/arch/x86/kernel/signal.c
@@ -212,6 +212,11 @@ do {									\
  * Set up a signal frame.
  */
 
+/* x86 ABI requires 16-byte alignment */
+#define FRAME_ALIGNMENT	16UL
+
+#define MAX_FRAME_PADDING	(FRAME_ALIGNMENT - 1)
+
 /*
  * Determine which stack to use..
  */
@@ -222,9 +227,9 @@ static unsigned long align_sigframe(unsigned long sp)
 	 * Align the stack pointer according to the i386 ABI,
 	 * i.e. so that on function entry ((sp + 4) & 15) == 0.
 	 */
-	sp = ((sp + 4) & -16ul) - 4;
+	sp = ((sp + 4) & -FRAME_ALIGNMENT) - 4;
 #else /* !CONFIG_X86_32 */
-	sp = round_down(sp, 16) - 8;
+	sp = round_down(sp, FRAME_ALIGNMENT) - 8;
 #endif
 	return sp;
 }
@@ -663,6 +668,56 @@ SYSCALL_DEFINE0(rt_sigreturn)
 	return 0;
 }
 
+/*
+ * There are four different struct types for signal frame: sigframe_ia32,
+ * rt_sigframe_ia32, rt_sigframe_x32, and rt_sigframe. Use the worst case
+ * -- the largest size. It means the size for 64-bit apps is a bit more
+ * than needed, but this keeps the code simple.
+ */
+#if defined(CONFIG_X86_32) || defined(CONFIG_IA32_EMULATION)
+# define MAX_FRAME_SIGINFO_UCTXT_SIZE	sizeof(struct sigframe_ia32)
+#else
+# define MAX_FRAME_SIGINFO_UCTXT_SIZE	sizeof(struct rt_sigframe)
+#endif
+
+/*
+ * The FP state frame contains an XSAVE buffer which must be 64-byte aligned.
+ * If a signal frame starts at an unaligned address, extra space is required.
+ * This is the max alignment padding, conservatively.
+ */
+#define MAX_XSAVE_PADDING	63UL
+
+/*
+ * The frame data is composed of the following areas and laid out as:
+ *
+ * -------------------------
+ * | alignment padding     |
+ * -------------------------
+ * | (f)xsave frame        |
+ * -------------------------
+ * | fsave header          |
+ * -------------------------
+ * | alignment padding     |
+ * -------------------------
+ * | siginfo + ucontext    |
+ * -------------------------
+ */
+
+/* max_frame_size tells userspace the worst case signal stack size. */
+static unsigned long __ro_after_init max_frame_size;
+
+void __init init_sigframe_size(void)
+{
+	max_frame_size = MAX_FRAME_SIGINFO_UCTXT_SIZE + MAX_FRAME_PADDING;
+
+	max_frame_size += fpu__get_fpstate_size() + MAX_XSAVE_PADDING;
+
+	/* Userspace expects an aligned size. */
+	max_frame_size = round_up(max_frame_size, FRAME_ALIGNMENT);
+
+	pr_info("max sigframe size: %lu\n", max_frame_size);
+}
+
 static inline int is_ia32_compat_frame(struct ksignal *ksig)
 {
 	return IS_ENABLED(CONFIG_IA32_EMULATION) &&
-- 
2.17.1

[PATCH v9 3/6] x86/elf: Support a new ELF aux vector AT_MINSIGSTKSZ

[Chang S. Bae]

Historically, signal.h defines MINSIGSTKSZ (2KB) and SIGSTKSZ (8KB), for
use by all architectures with sigaltstack(2). Over time, the hardware state
size grew, but these constants did not evolve. Today, literal use of these
constants on several architectures may result in signal stack overflow, and
thus user data corruption.

A few years ago, the ARM team addressed this issue by establishing
getauxval(AT_MINSIGSTKSZ). This enables the kernel to supply at runtime
value that is an appropriate replacement on the current and future
hardware.

Add getauxval(AT_MINSIGSTKSZ) support to x86, analogous to the support
added for ARM in commit 94b07c1f8c39 ("arm64: signal: Report signal frame
size to userspace via auxv").

Also, include a documentation to describe x86-specific auxiliary vectors.

Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Reviewed-by: Len Brown <len.brown@intel.com>
Cc: H.J. Lu <hjl.tools@gmail.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: Dave Martin <Dave.Martin@arm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: x86@kernel.org
Cc: libc-alpha@sourceware.org
Cc: linux-arch@vger.kernel.org
Cc: linux-api@vger.kernel.org
Cc: linux-doc@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
---
Changes from v7:
* Delegated the bugfix notion to the other patch.

Changes from v6:
* Revised the documentation and fixed the build issue. (Borislav Petkov)
* Fixed the vertical alignment of '\'. (Borislav Petkov)

Changes from v5:
* Added a documentation.
---
 Documentation/x86/elf_auxvec.rst   | 53 ++++++++++++++++++++++++++++++
 Documentation/x86/index.rst        |  1 +
 arch/x86/include/asm/elf.h         |  4 +++
 arch/x86/include/uapi/asm/auxvec.h |  4 +--
 arch/x86/kernel/signal.c           |  5 +++
 5 files changed, 65 insertions(+), 2 deletions(-)
 create mode 100644 Documentation/x86/elf_auxvec.rst

diff --git a/Documentation/x86/elf_auxvec.rst b/Documentation/x86/elf_auxvec.rst
new file mode 100644
index 000000000000..6c75b26f5efb
--- /dev/null
+++ b/Documentation/x86/elf_auxvec.rst
@@ -0,0 +1,53 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+==================================
+x86-specific ELF Auxiliary Vectors
+==================================
+
+This document describes the semantics of the x86 auxiliary vectors.
+
+Introduction
+============
+
+ELF Auxiliary vectors enable the kernel to efficiently provide
+configuration specific parameters to userspace. In this example, a program
+allocates an alternate stack based on the kernel-provided size::
+
+   #include <sys/auxv.h>
+   #include <elf.h>
+   #include <signal.h>
+   #include <stdlib.h>
+   #include <assert.h>
+   #include <err.h>
+
+   #ifndef AT_MINSIGSTKSZ
+   #define AT_MINSIGSTKSZ	51
+   #endif
+
+   ....
+   stack_t ss;
+
+   ss.ss_sp = malloc(ss.ss_size);
+   assert(ss.ss_sp);
+
+   ss.ss_size = getauxval(AT_MINSIGSTKSZ) + SIGSTKSZ;
+   ss.ss_flags = 0;
+
+   if (sigaltstack(&ss, NULL))
+        err(1, "sigaltstack");
+
+
+The exposed auxiliary vectors
+=============================
+
+AT_SYSINFO is used for locating the vsyscall entry point.  It is not
+exported on 64-bit mode.
+
+AT_SYSINFO_EHDR is the start address of the page containing the vDSO.
+
+AT_MINSIGSTKSZ denotes the minimum stack size required by the kernel to
+deliver a signal to user-space.  AT_MINSIGSTKSZ comprehends the space
+consumed by the kernel to accommodate the user context for the current
+hardware configuration.  It does not comprehend subsequent user-space stack
+consumption, which must be added by the user.  (e.g. Above, user-space adds
+SIGSTKSZ to AT_MINSIGSTKSZ.)
diff --git a/Documentation/x86/index.rst b/Documentation/x86/index.rst
index 4693e192b447..d58614d5cde6 100644
--- a/Documentation/x86/index.rst
+++ b/Documentation/x86/index.rst
@@ -35,3 +35,4 @@ x86-specific Documentation
    sva
    sgx
    features
+   elf_auxvec
diff --git a/arch/x86/include/asm/elf.h b/arch/x86/include/asm/elf.h
index 7d7500806af8..29fea180a665 100644
--- a/arch/x86/include/asm/elf.h
+++ b/arch/x86/include/asm/elf.h
@@ -312,6 +312,7 @@ do {									\
 		NEW_AUX_ENT(AT_SYSINFO,	VDSO_ENTRY);			\
 		NEW_AUX_ENT(AT_SYSINFO_EHDR, VDSO_CURRENT_BASE);	\
 	}								\
+	NEW_AUX_ENT(AT_MINSIGSTKSZ, get_sigframe_size());		\
 } while (0)
 
 /*
@@ -328,6 +329,7 @@ extern unsigned long task_size_32bit(void);
 extern unsigned long task_size_64bit(int full_addr_space);
 extern unsigned long get_mmap_base(int is_legacy);
 extern bool mmap_address_hint_valid(unsigned long addr, unsigned long len);
+extern unsigned long get_sigframe_size(void);
 
 #ifdef CONFIG_X86_32
 
@@ -349,6 +351,7 @@ do {									\
 	if (vdso64_enabled)						\
 		NEW_AUX_ENT(AT_SYSINFO_EHDR,				\
 			    (unsigned long __force)current->mm->context.vdso); \
+	NEW_AUX_ENT(AT_MINSIGSTKSZ, get_sigframe_size());		\
 } while (0)
 
 /* As a historical oddity, the x32 and x86_64 vDSOs are controlled together. */
@@ -357,6 +360,7 @@ do {									\
 	if (vdso64_enabled)						\
 		NEW_AUX_ENT(AT_SYSINFO_EHDR,				\
 			    (unsigned long __force)current->mm->context.vdso); \
+	NEW_AUX_ENT(AT_MINSIGSTKSZ, get_sigframe_size());		\
 } while (0)
 
 #define AT_SYSINFO		32
diff --git a/arch/x86/include/uapi/asm/auxvec.h b/arch/x86/include/uapi/asm/auxvec.h
index 580e3c567046..6beb55bbefa4 100644
--- a/arch/x86/include/uapi/asm/auxvec.h
+++ b/arch/x86/include/uapi/asm/auxvec.h
@@ -12,9 +12,9 @@
 
 /* entries in ARCH_DLINFO: */
 #if defined(CONFIG_IA32_EMULATION) || !defined(CONFIG_X86_64)
-# define AT_VECTOR_SIZE_ARCH 2
+# define AT_VECTOR_SIZE_ARCH 3
 #else /* else it's non-compat x86-64 */
-# define AT_VECTOR_SIZE_ARCH 1
+# define AT_VECTOR_SIZE_ARCH 2
 #endif
 
 #endif /* _ASM_X86_AUXVEC_H */
diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c
index 689a4b6dd18f..86e53868159a 100644
--- a/arch/x86/kernel/signal.c
+++ b/arch/x86/kernel/signal.c
@@ -718,6 +718,11 @@ void __init init_sigframe_size(void)
 	pr_info("max sigframe size: %lu\n", max_frame_size);
 }
 
+unsigned long get_sigframe_size(void)
+{
+	return max_frame_size;
+}
+
 static inline int is_ia32_compat_frame(struct ksignal *ksig)
 {
 	return IS_ENABLED(CONFIG_IA32_EMULATION) &&
-- 
2.17.1

[PATCH v9 4/6] selftest/sigaltstack: Use the AT_MINSIGSTKSZ aux vector if available

[Chang S. Bae]

The SIGSTKSZ constant may not represent enough stack size in some
architectures as the hardware state size grows.

Use getauxval(AT_MINSIGSTKSZ) to increase the stack size.

Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Reviewed-by: Len Brown <len.brown@intel.com>
Cc: linux-kselftest@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
---
Changes from v5:
* Added as a new patch.
---
 tools/testing/selftests/sigaltstack/sas.c | 20 +++++++++++++++-----
 1 file changed, 15 insertions(+), 5 deletions(-)

diff --git a/tools/testing/selftests/sigaltstack/sas.c b/tools/testing/selftests/sigaltstack/sas.c
index 8934a3766d20..c53b070755b6 100644
--- a/tools/testing/selftests/sigaltstack/sas.c
+++ b/tools/testing/selftests/sigaltstack/sas.c
@@ -17,6 +17,7 @@
 #include <string.h>
 #include <assert.h>
 #include <errno.h>
+#include <sys/auxv.h>
 
 #include "../kselftest.h"
 
@@ -24,6 +25,11 @@
 #define SS_AUTODISARM  (1U << 31)
 #endif
 
+#ifndef AT_MINSIGSTKSZ
+#define AT_MINSIGSTKSZ	51
+#endif
+
+static unsigned int stack_size;
 static void *sstack, *ustack;
 static ucontext_t uc, sc;
 static const char *msg = "[OK]\tStack preserved";
@@ -47,7 +53,7 @@ void my_usr1(int sig, siginfo_t *si, void *u)
 #endif
 
 	if (sp < (unsigned long)sstack ||
-			sp >= (unsigned long)sstack + SIGSTKSZ) {
+			sp >= (unsigned long)sstack + stack_size) {
 		ksft_exit_fail_msg("SP is not on sigaltstack\n");
 	}
 	/* put some data on stack. other sighandler will try to overwrite it */
@@ -108,6 +114,10 @@ int main(void)
 	stack_t stk;
 	int err;
 
+	/* Make sure more than the required minimum. */
+	stack_size = getauxval(AT_MINSIGSTKSZ) + SIGSTKSZ;
+	ksft_print_msg("[NOTE]\tthe stack size is %lu\n", stack_size);
+
 	ksft_print_header();
 	ksft_set_plan(3);
 
@@ -117,7 +127,7 @@ int main(void)
 	sigaction(SIGUSR1, &act, NULL);
 	act.sa_sigaction = my_usr2;
 	sigaction(SIGUSR2, &act, NULL);
-	sstack = mmap(NULL, SIGSTKSZ, PROT_READ | PROT_WRITE,
+	sstack = mmap(NULL, stack_size, PROT_READ | PROT_WRITE,
 		      MAP_PRIVATE | MAP_ANONYMOUS | MAP_STACK, -1, 0);
 	if (sstack == MAP_FAILED) {
 		ksft_exit_fail_msg("mmap() - %s\n", strerror(errno));
@@ -139,7 +149,7 @@ int main(void)
 	}
 
 	stk.ss_sp = sstack;
-	stk.ss_size = SIGSTKSZ;
+	stk.ss_size = stack_size;
 	stk.ss_flags = SS_ONSTACK | SS_AUTODISARM;
 	err = sigaltstack(&stk, NULL);
 	if (err) {
@@ -161,7 +171,7 @@ int main(void)
 		}
 	}
 
-	ustack = mmap(NULL, SIGSTKSZ, PROT_READ | PROT_WRITE,
+	ustack = mmap(NULL, stack_size, PROT_READ | PROT_WRITE,
 		      MAP_PRIVATE | MAP_ANONYMOUS | MAP_STACK, -1, 0);
 	if (ustack == MAP_FAILED) {
 		ksft_exit_fail_msg("mmap() - %s\n", strerror(errno));
@@ -170,7 +180,7 @@ int main(void)
 	getcontext(&uc);
 	uc.uc_link = NULL;
 	uc.uc_stack.ss_sp = ustack;
-	uc.uc_stack.ss_size = SIGSTKSZ;
+	uc.uc_stack.ss_size = stack_size;
 	makecontext(&uc, switch_fn, 0);
 	raise(SIGUSR1);
 
-- 
2.17.1

[PATCH v9 5/6] x86/signal: Detect and prevent an alternate signal stack overflow

[Chang S. Bae]

The kernel pushes context on to the userspace stack to prepare for the
user's signal handler. When the user has supplied an alternate signal
stack, via sigaltstack(2), it is easy for the kernel to verify that the
stack size is sufficient for the current hardware context.

Check if writing the hardware context to the alternate stack will exceed
it's size. If yes, then instead of corrupting user-data and proceeding with
the original signal handler, an immediate SIGSEGV signal is delivered.

Refactor the stack pointer check code from on_sig_stack() and use the new
helper.

While the kernel allows new source code to discover and use a sufficient
alternate signal stack size, this check is still necessary to protect
binaries with insufficient alternate signal stack size from data
corruption.

Reported-by: Florian Weimer <fweimer@redhat.com>
Fixes: c2bc11f10a39 ("x86, AVX-512: Enable AVX-512 States Context Switch")
Suggested-by: Jann Horn <jannh@google.com>
Suggested-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Reviewed-by: Len Brown <len.brown@intel.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Jann Horn <jannh@google.com>
Cc: x86@kernel.org
Cc: linux-kernel@vger.kernel.org
Link: https://bugzilla.kernel.org/show_bug.cgi?id=153531
---
Changes from v8:
* Added a "\n" at the end of the sigaltstack overflow message. (Borislav
  Petkov)

Changes from v7:
* Separated the notion for entering altstack from a nested signal on the
  altstack. (Andy Lutomirski)
* Added the message for sigalstack overflow. (Andy Lutomirski)
* Refactored on_sig_stack(). (Borislav Petkov)
* Included the "Fixes" tag and the bugzilla link as this patch fixes the
  kernel behavior.

Changes from v5:
* Fixed the overflow check. (Andy Lutomirski)
* Updated the changelog.

Changes from v3:
* Updated the changelog (Borislav Petkov)

Changes from v2:
* Simplified the implementation (Jann Horn)
---
 arch/x86/kernel/signal.c     | 24 ++++++++++++++++++++----
 include/linux/sched/signal.h | 19 ++++++++++++-------
 2 files changed, 32 insertions(+), 11 deletions(-)

diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c
index 86e53868159a..2ddcf2165bcb 100644
--- a/arch/x86/kernel/signal.c
+++ b/arch/x86/kernel/signal.c
@@ -239,10 +239,11 @@ get_sigframe(struct k_sigaction *ka, struct pt_regs *regs, size_t frame_size,
 	     void __user **fpstate)
 {
 	/* Default to using normal stack */
+	bool nested_altstack = on_sig_stack(regs->sp);
+	bool entering_altstack = false;
 	unsigned long math_size = 0;
 	unsigned long sp = regs->sp;
 	unsigned long buf_fx = 0;
-	int onsigstack = on_sig_stack(sp);
 	int ret;
 
 	/* redzone */
@@ -251,15 +252,23 @@ get_sigframe(struct k_sigaction *ka, struct pt_regs *regs, size_t frame_size,
 
 	/* This is the X/Open sanctioned signal stack switching.  */
 	if (ka->sa.sa_flags & SA_ONSTACK) {
-		if (sas_ss_flags(sp) == 0)
+		/*
+		 * This checks nested_altstack via sas_ss_flags(). Sensible
+		 * programs use SS_AUTODISARM, which disables that check, and
+		 * programs that don't use SS_AUTODISARM get compatible.
+		 */
+		if (sas_ss_flags(sp) == 0) {
 			sp = current->sas_ss_sp + current->sas_ss_size;
+			entering_altstack = true;
+		}
 	} else if (IS_ENABLED(CONFIG_X86_32) &&
-		   !onsigstack &&
+		   !nested_altstack &&
 		   regs->ss != __USER_DS &&
 		   !(ka->sa.sa_flags & SA_RESTORER) &&
 		   ka->sa.sa_restorer) {
 		/* This is the legacy signal stack switching. */
 		sp = (unsigned long) ka->sa.sa_restorer;
+		entering_altstack = true;
 	}
 
 	sp = fpu__alloc_mathframe(sp, IS_ENABLED(CONFIG_X86_32),
@@ -272,8 +281,15 @@ get_sigframe(struct k_sigaction *ka, struct pt_regs *regs, size_t frame_size,
 	 * If we are on the alternate signal stack and would overflow it, don't.
 	 * Return an always-bogus address instead so we will die with SIGSEGV.
 	 */
-	if (onsigstack && !likely(on_sig_stack(sp)))
+	if (unlikely((nested_altstack || entering_altstack) &&
+		     !__on_sig_stack(sp))) {
+
+		if (show_unhandled_signals && printk_ratelimit())
+			pr_info("%s[%d] overflowed sigaltstack\n",
+				current->comm, task_pid_nr(current));
+
 		return (void __user *)-1L;
+	}
 
 	/* save i387 and extended state */
 	ret = copy_fpstate_to_sigframe(*fpstate, (void __user *)buf_fx, math_size);
diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
index 3f6a0fcaa10c..ae60f838ebb9 100644
--- a/include/linux/sched/signal.h
+++ b/include/linux/sched/signal.h
@@ -537,6 +537,17 @@ static inline int kill_cad_pid(int sig, int priv)
 #define SEND_SIG_NOINFO ((struct kernel_siginfo *) 0)
 #define SEND_SIG_PRIV	((struct kernel_siginfo *) 1)
 
+static inline int __on_sig_stack(unsigned long sp)
+{
+#ifdef CONFIG_STACK_GROWSUP
+	return sp >= current->sas_ss_sp &&
+		sp - current->sas_ss_sp < current->sas_ss_size;
+#else
+	return sp > current->sas_ss_sp &&
+		sp - current->sas_ss_sp <= current->sas_ss_size;
+#endif
+}
+
 /*
  * True if we are on the alternate signal stack.
  */
@@ -554,13 +565,7 @@ static inline int on_sig_stack(unsigned long sp)
 	if (current->sas_ss_flags & SS_AUTODISARM)
 		return 0;
 
-#ifdef CONFIG_STACK_GROWSUP
-	return sp >= current->sas_ss_sp &&
-		sp - current->sas_ss_sp < current->sas_ss_size;
-#else
-	return sp > current->sas_ss_sp &&
-		sp - current->sas_ss_sp <= current->sas_ss_size;
-#endif
+	return __on_sig_stack(sp);
 }
 
 static inline int sas_ss_flags(unsigned long sp)
-- 
2.17.1

[PATCH v9 6/6] selftest/x86/signal: Include test cases for validating sigaltstack

[Chang S. Bae]

The test measures the kernel's signal delivery with different (enough vs.
insufficient) stack sizes.

Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Reviewed-by: Len Brown <len.brown@intel.com>
Cc: x86@kernel.org
Cc: linux-kselftest@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
---
Changes from v3:
* Revised test messages again (Borislav Petkov)

Changes from v2:
* Revised test messages (Borislav Petkov)
---
 tools/testing/selftests/x86/Makefile      |   2 +-
 tools/testing/selftests/x86/sigaltstack.c | 128 ++++++++++++++++++++++
 2 files changed, 129 insertions(+), 1 deletion(-)
 create mode 100644 tools/testing/selftests/x86/sigaltstack.c

diff --git a/tools/testing/selftests/x86/Makefile b/tools/testing/selftests/x86/Makefile
index 333980375bc7..65bba2ae86ee 100644
--- a/tools/testing/selftests/x86/Makefile
+++ b/tools/testing/selftests/x86/Makefile
@@ -13,7 +13,7 @@ CAN_BUILD_WITH_NOPIE := $(shell ./check_cc.sh $(CC) trivial_program.c -no-pie)
 TARGETS_C_BOTHBITS := single_step_syscall sysret_ss_attrs syscall_nt test_mremap_vdso \
 			check_initial_reg_state sigreturn iopl ioperm \
 			test_vsyscall mov_ss_trap \
-			syscall_arg_fault fsgsbase_restore
+			syscall_arg_fault fsgsbase_restore sigaltstack
 TARGETS_C_32BIT_ONLY := entry_from_vm86 test_syscall_vdso unwind_vdso \
 			test_FCMOV test_FCOMI test_FISTTP \
 			vdso_restorer
diff --git a/tools/testing/selftests/x86/sigaltstack.c b/tools/testing/selftests/x86/sigaltstack.c
new file mode 100644
index 000000000000..f689af75e979
--- /dev/null
+++ b/tools/testing/selftests/x86/sigaltstack.c
@@ -0,0 +1,128 @@
+// SPDX-License-Identifier: GPL-2.0-only
+
+#define _GNU_SOURCE
+#include <signal.h>
+#include <stdio.h>
+#include <stdbool.h>
+#include <string.h>
+#include <err.h>
+#include <errno.h>
+#include <limits.h>
+#include <sys/mman.h>
+#include <sys/auxv.h>
+#include <sys/prctl.h>
+#include <sys/resource.h>
+#include <setjmp.h>
+
+/* sigaltstack()-enforced minimum stack */
+#define ENFORCED_MINSIGSTKSZ	2048
+
+#ifndef AT_MINSIGSTKSZ
+#  define AT_MINSIGSTKSZ	51
+#endif
+
+static int nerrs;
+
+static bool sigalrm_expected;
+
+static unsigned long at_minstack_size;
+
+static void sethandler(int sig, void (*handler)(int, siginfo_t *, void *),
+		       int flags)
+{
+	struct sigaction sa;
+
+	memset(&sa, 0, sizeof(sa));
+	sa.sa_sigaction = handler;
+	sa.sa_flags = SA_SIGINFO | flags;
+	sigemptyset(&sa.sa_mask);
+	if (sigaction(sig, &sa, 0))
+		err(1, "sigaction");
+}
+
+static void clearhandler(int sig)
+{
+	struct sigaction sa;
+
+	memset(&sa, 0, sizeof(sa));
+	sa.sa_handler = SIG_DFL;
+	sigemptyset(&sa.sa_mask);
+	if (sigaction(sig, &sa, 0))
+		err(1, "sigaction");
+}
+
+static int setup_altstack(void *start, unsigned long size)
+{
+	stack_t ss;
+
+	memset(&ss, 0, sizeof(ss));
+	ss.ss_size = size;
+	ss.ss_sp = start;
+
+	return sigaltstack(&ss, NULL);
+}
+
+static jmp_buf jmpbuf;
+
+static void sigsegv(int sig, siginfo_t *info, void *ctx_void)
+{
+	if (sigalrm_expected) {
+		printf("[FAIL]\tWrong signal delivered: SIGSEGV (expected SIGALRM).");
+		nerrs++;
+	} else {
+		printf("[OK]\tSIGSEGV signal delivered.\n");
+	}
+
+	siglongjmp(jmpbuf, 1);
+}
+
+static void sigalrm(int sig, siginfo_t *info, void *ctx_void)
+{
+	if (!sigalrm_expected) {
+		printf("[FAIL]\tWrong signal delivered: SIGALRM (expected SIGSEGV).");
+		nerrs++;
+	} else {
+		printf("[OK]\tSIGALRM signal delivered.\n");
+	}
+}
+
+static void test_sigaltstack(void *altstack, unsigned long size)
+{
+	if (setup_altstack(altstack, size))
+		err(1, "sigaltstack()");
+
+	sigalrm_expected = (size > at_minstack_size) ? true : false;
+
+	sethandler(SIGSEGV, sigsegv, 0);
+	sethandler(SIGALRM, sigalrm, SA_ONSTACK);
+
+	if (!sigsetjmp(jmpbuf, 1)) {
+		printf("[RUN]\tTest an alternate signal stack of %ssufficient size.\n",
+		       sigalrm_expected ? "" : "in");
+		printf("\tRaise SIGALRM. %s is expected to be delivered.\n",
+		       sigalrm_expected ? "It" : "SIGSEGV");
+		raise(SIGALRM);
+	}
+
+	clearhandler(SIGALRM);
+	clearhandler(SIGSEGV);
+}
+
+int main(void)
+{
+	void *altstack;
+
+	at_minstack_size = getauxval(AT_MINSIGSTKSZ);
+
+	altstack = mmap(NULL, at_minstack_size + SIGSTKSZ, PROT_READ | PROT_WRITE,
+			MAP_PRIVATE | MAP_ANONYMOUS | MAP_STACK, -1, 0);
+	if (altstack == MAP_FAILED)
+		err(1, "mmap()");
+
+	if ((ENFORCED_MINSIGSTKSZ + 1) < at_minstack_size)
+		test_sigaltstack(altstack, ENFORCED_MINSIGSTKSZ + 1);
+
+	test_sigaltstack(altstack, at_minstack_size + SIGSTKSZ);
+
+	return nerrs == 0 ? 0 : 1;
+}
-- 
2.17.1

Re: [PATCH v9 0/6] Improve Minimum Alternate Stack Size

[Thomas Gleixner]

On Tue, May 18 2021 at 13:03, Chang S. Bae wrote:
> During signal entry, the kernel pushes data onto the normal userspace
> stack. On x86, the data pushed onto the user stack includes XSAVE state,
> which has grown over time as new features and larger registers have been
> added to the architecture.
>
> MINSIGSTKSZ is a constant provided in the kernel signal.h headers and
> typically distributed in lib-dev(el) packages, e.g. [1]. Its value is
> compiled into programs and is part of the user/kernel ABI. The MINSIGSTKSZ
> constant indicates to userspace how much data the kernel expects to push on
> the user stack, [2][3].
>
> However, this constant is much too small and does not reflect recent
> additions to the architecture. For instance, when AVX-512 states are in
> use, the signal frame size can be 3.5KB while MINSIGSTKSZ remains 2KB.
>
> The bug report [4] explains this as an ABI issue. The small MINSIGSTKSZ can
> cause user stack overflow when delivering a signal.

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

[tip: x86/fpu] selftest/x86/signal: Include test cases for validating sigaltstack

[tip-bot2 for Chang S. Bae]

The following commit has been merged into the x86/fpu branch of tip:

Commit-ID:     8919f07276991c7bf0d0802f0356331c5c62f7a2
Gitweb:        https://git.kernel.org/tip/8919f07276991c7bf0d0802f0356331c5c62f7a2
Author:        Chang S. Bae <chang.seok.bae@intel.com>
AuthorDate:    Tue, 18 May 2021 13:03:20 -07:00
Committer:     Borislav Petkov <bp@suse.de>
CommitterDate: Wed, 19 May 2021 12:45:07 +02:00

selftest/x86/signal: Include test cases for validating sigaltstack

The test measures the kernel's signal delivery with different (enough vs.
insufficient) stack sizes.

Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Len Brown <len.brown@intel.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20210518200320.17239-7-chang.seok.bae@intel.com
---
 tools/testing/selftests/x86/Makefile      |   2 +-
 tools/testing/selftests/x86/sigaltstack.c | 128 +++++++++++++++++++++-
 2 files changed, 129 insertions(+), 1 deletion(-)
 create mode 100644 tools/testing/selftests/x86/sigaltstack.c

diff --git a/tools/testing/selftests/x86/Makefile b/tools/testing/selftests/x86/Makefile
index 3339803..65bba2a 100644
--- a/tools/testing/selftests/x86/Makefile
+++ b/tools/testing/selftests/x86/Makefile
@@ -13,7 +13,7 @@ CAN_BUILD_WITH_NOPIE := $(shell ./check_cc.sh $(CC) trivial_program.c -no-pie)
 TARGETS_C_BOTHBITS := single_step_syscall sysret_ss_attrs syscall_nt test_mremap_vdso \
 			check_initial_reg_state sigreturn iopl ioperm \
 			test_vsyscall mov_ss_trap \
-			syscall_arg_fault fsgsbase_restore
+			syscall_arg_fault fsgsbase_restore sigaltstack
 TARGETS_C_32BIT_ONLY := entry_from_vm86 test_syscall_vdso unwind_vdso \
 			test_FCMOV test_FCOMI test_FISTTP \
 			vdso_restorer
diff --git a/tools/testing/selftests/x86/sigaltstack.c b/tools/testing/selftests/x86/sigaltstack.c
new file mode 100644
index 0000000..f689af7
--- /dev/null
+++ b/tools/testing/selftests/x86/sigaltstack.c
@@ -0,0 +1,128 @@
+// SPDX-License-Identifier: GPL-2.0-only
+
+#define _GNU_SOURCE
+#include <signal.h>
+#include <stdio.h>
+#include <stdbool.h>
+#include <string.h>
+#include <err.h>
+#include <errno.h>
+#include <limits.h>
+#include <sys/mman.h>
+#include <sys/auxv.h>
+#include <sys/prctl.h>
+#include <sys/resource.h>
+#include <setjmp.h>
+
+/* sigaltstack()-enforced minimum stack */
+#define ENFORCED_MINSIGSTKSZ	2048
+
+#ifndef AT_MINSIGSTKSZ
+#  define AT_MINSIGSTKSZ	51
+#endif
+
+static int nerrs;
+
+static bool sigalrm_expected;
+
+static unsigned long at_minstack_size;
+
+static void sethandler(int sig, void (*handler)(int, siginfo_t *, void *),
+		       int flags)
+{
+	struct sigaction sa;
+
+	memset(&sa, 0, sizeof(sa));
+	sa.sa_sigaction = handler;
+	sa.sa_flags = SA_SIGINFO | flags;
+	sigemptyset(&sa.sa_mask);
+	if (sigaction(sig, &sa, 0))
+		err(1, "sigaction");
+}
+
+static void clearhandler(int sig)
+{
+	struct sigaction sa;
+
+	memset(&sa, 0, sizeof(sa));
+	sa.sa_handler = SIG_DFL;
+	sigemptyset(&sa.sa_mask);
+	if (sigaction(sig, &sa, 0))
+		err(1, "sigaction");
+}
+
+static int setup_altstack(void *start, unsigned long size)
+{
+	stack_t ss;
+
+	memset(&ss, 0, sizeof(ss));
+	ss.ss_size = size;
+	ss.ss_sp = start;
+
+	return sigaltstack(&ss, NULL);
+}
+
+static jmp_buf jmpbuf;
+
+static void sigsegv(int sig, siginfo_t *info, void *ctx_void)
+{
+	if (sigalrm_expected) {
+		printf("[FAIL]\tWrong signal delivered: SIGSEGV (expected SIGALRM).");
+		nerrs++;
+	} else {
+		printf("[OK]\tSIGSEGV signal delivered.\n");
+	}
+
+	siglongjmp(jmpbuf, 1);
+}
+
+static void sigalrm(int sig, siginfo_t *info, void *ctx_void)
+{
+	if (!sigalrm_expected) {
+		printf("[FAIL]\tWrong signal delivered: SIGALRM (expected SIGSEGV).");
+		nerrs++;
+	} else {
+		printf("[OK]\tSIGALRM signal delivered.\n");
+	}
+}
+
+static void test_sigaltstack(void *altstack, unsigned long size)
+{
+	if (setup_altstack(altstack, size))
+		err(1, "sigaltstack()");
+
+	sigalrm_expected = (size > at_minstack_size) ? true : false;
+
+	sethandler(SIGSEGV, sigsegv, 0);
+	sethandler(SIGALRM, sigalrm, SA_ONSTACK);
+
+	if (!sigsetjmp(jmpbuf, 1)) {
+		printf("[RUN]\tTest an alternate signal stack of %ssufficient size.\n",
+		       sigalrm_expected ? "" : "in");
+		printf("\tRaise SIGALRM. %s is expected to be delivered.\n",
+		       sigalrm_expected ? "It" : "SIGSEGV");
+		raise(SIGALRM);
+	}
+
+	clearhandler(SIGALRM);
+	clearhandler(SIGSEGV);
+}
+
+int main(void)
+{
+	void *altstack;
+
+	at_minstack_size = getauxval(AT_MINSIGSTKSZ);
+
+	altstack = mmap(NULL, at_minstack_size + SIGSTKSZ, PROT_READ | PROT_WRITE,
+			MAP_PRIVATE | MAP_ANONYMOUS | MAP_STACK, -1, 0);
+	if (altstack == MAP_FAILED)
+		err(1, "mmap()");
+
+	if ((ENFORCED_MINSIGSTKSZ + 1) < at_minstack_size)
+		test_sigaltstack(altstack, ENFORCED_MINSIGSTKSZ + 1);
+
+	test_sigaltstack(altstack, at_minstack_size + SIGSTKSZ);
+
+	return nerrs == 0 ? 0 : 1;
+}

[tip: x86/fpu] selftest/sigaltstack: Use the AT_MINSIGSTKSZ aux vector if available

[tip-bot2 for Chang S. Bae]

The following commit has been merged into the x86/fpu branch of tip:

Commit-ID:     bdf6c8b84a4fa726c382ef6d3518f3ae123a7ebd
Gitweb:        https://git.kernel.org/tip/bdf6c8b84a4fa726c382ef6d3518f3ae123a7ebd
Author:        Chang S. Bae <chang.seok.bae@intel.com>
AuthorDate:    Tue, 18 May 2021 13:03:18 -07:00
Committer:     Borislav Petkov <bp@suse.de>
CommitterDate: Wed, 19 May 2021 12:38:17 +02:00

selftest/sigaltstack: Use the AT_MINSIGSTKSZ aux vector if available

The SIGSTKSZ constant may not represent enough stack size in some
architectures as the hardware state size grows.

Use getauxval(AT_MINSIGSTKSZ) to increase the stack size.

Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Len Brown <len.brown@intel.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20210518200320.17239-5-chang.seok.bae@intel.com
---
 tools/testing/selftests/sigaltstack/sas.c | 20 +++++++++++++++-----
 1 file changed, 15 insertions(+), 5 deletions(-)

diff --git a/tools/testing/selftests/sigaltstack/sas.c b/tools/testing/selftests/sigaltstack/sas.c
index 8934a37..c53b070 100644
--- a/tools/testing/selftests/sigaltstack/sas.c
+++ b/tools/testing/selftests/sigaltstack/sas.c
@@ -17,6 +17,7 @@
 #include <string.h>
 #include <assert.h>
 #include <errno.h>
+#include <sys/auxv.h>
 
 #include "../kselftest.h"
 
@@ -24,6 +25,11 @@
 #define SS_AUTODISARM  (1U << 31)
 #endif
 
+#ifndef AT_MINSIGSTKSZ
+#define AT_MINSIGSTKSZ	51
+#endif
+
+static unsigned int stack_size;
 static void *sstack, *ustack;
 static ucontext_t uc, sc;
 static const char *msg = "[OK]\tStack preserved";
@@ -47,7 +53,7 @@ void my_usr1(int sig, siginfo_t *si, void *u)
 #endif
 
 	if (sp < (unsigned long)sstack ||
-			sp >= (unsigned long)sstack + SIGSTKSZ) {
+			sp >= (unsigned long)sstack + stack_size) {
 		ksft_exit_fail_msg("SP is not on sigaltstack\n");
 	}
 	/* put some data on stack. other sighandler will try to overwrite it */
@@ -108,6 +114,10 @@ int main(void)
 	stack_t stk;
 	int err;
 
+	/* Make sure more than the required minimum. */
+	stack_size = getauxval(AT_MINSIGSTKSZ) + SIGSTKSZ;
+	ksft_print_msg("[NOTE]\tthe stack size is %lu\n", stack_size);
+
 	ksft_print_header();
 	ksft_set_plan(3);
 
@@ -117,7 +127,7 @@ int main(void)
 	sigaction(SIGUSR1, &act, NULL);
 	act.sa_sigaction = my_usr2;
 	sigaction(SIGUSR2, &act, NULL);
-	sstack = mmap(NULL, SIGSTKSZ, PROT_READ | PROT_WRITE,
+	sstack = mmap(NULL, stack_size, PROT_READ | PROT_WRITE,
 		      MAP_PRIVATE | MAP_ANONYMOUS | MAP_STACK, -1, 0);
 	if (sstack == MAP_FAILED) {
 		ksft_exit_fail_msg("mmap() - %s\n", strerror(errno));
@@ -139,7 +149,7 @@ int main(void)
 	}
 
 	stk.ss_sp = sstack;
-	stk.ss_size = SIGSTKSZ;
+	stk.ss_size = stack_size;
 	stk.ss_flags = SS_ONSTACK | SS_AUTODISARM;
 	err = sigaltstack(&stk, NULL);
 	if (err) {
@@ -161,7 +171,7 @@ int main(void)
 		}
 	}
 
-	ustack = mmap(NULL, SIGSTKSZ, PROT_READ | PROT_WRITE,
+	ustack = mmap(NULL, stack_size, PROT_READ | PROT_WRITE,
 		      MAP_PRIVATE | MAP_ANONYMOUS | MAP_STACK, -1, 0);
 	if (ustack == MAP_FAILED) {
 		ksft_exit_fail_msg("mmap() - %s\n", strerror(errno));
@@ -170,7 +180,7 @@ int main(void)
 	getcontext(&uc);
 	uc.uc_link = NULL;
 	uc.uc_stack.ss_sp = ustack;
-	uc.uc_stack.ss_size = SIGSTKSZ;
+	uc.uc_stack.ss_size = stack_size;
 	makecontext(&uc, switch_fn, 0);
 	raise(SIGUSR1);
 

[tip: x86/fpu] x86/elf: Support a new ELF aux vector AT_MINSIGSTKSZ

[tip-bot2 for Chang S. Bae]

The following commit has been merged into the x86/fpu branch of tip:

Commit-ID:     1c33bb0507508af24fd754dd7123bd8e997fab2f
Gitweb:        https://git.kernel.org/tip/1c33bb0507508af24fd754dd7123bd8e997fab2f
Author:        Chang S. Bae <chang.seok.bae@intel.com>
AuthorDate:    Tue, 18 May 2021 13:03:17 -07:00
Committer:     Borislav Petkov <bp@suse.de>
CommitterDate: Wed, 19 May 2021 12:18:45 +02:00

x86/elf: Support a new ELF aux vector AT_MINSIGSTKSZ

Historically, signal.h defines MINSIGSTKSZ (2KB) and SIGSTKSZ (8KB), for
use by all architectures with sigaltstack(2). Over time, the hardware state
size grew, but these constants did not evolve. Today, literal use of these
constants on several architectures may result in signal stack overflow, and
thus user data corruption.

A few years ago, the ARM team addressed this issue by establishing
getauxval(AT_MINSIGSTKSZ). This enables the kernel to supply a value
at runtime that is an appropriate replacement on current and future
hardware.

Add getauxval(AT_MINSIGSTKSZ) support to x86, analogous to the support
added for ARM in

  94b07c1f8c39 ("arm64: signal: Report signal frame size to userspace via auxv").

Also, include a documentation to describe x86-specific auxiliary vectors.

Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Len Brown <len.brown@intel.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20210518200320.17239-4-chang.seok.bae@intel.com
---
 Documentation/x86/elf_auxvec.rst   | 53 +++++++++++++++++++++++++++++-
 Documentation/x86/index.rst        |  1 +-
 arch/x86/include/asm/elf.h         |  4 ++-
 arch/x86/include/uapi/asm/auxvec.h |  4 +-
 arch/x86/kernel/signal.c           |  5 +++-
 5 files changed, 65 insertions(+), 2 deletions(-)
 create mode 100644 Documentation/x86/elf_auxvec.rst

diff --git a/Documentation/x86/elf_auxvec.rst b/Documentation/x86/elf_auxvec.rst
new file mode 100644
index 0000000..18e4744
--- /dev/null
+++ b/Documentation/x86/elf_auxvec.rst
@@ -0,0 +1,53 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+==================================
+x86-specific ELF Auxiliary Vectors
+==================================
+
+This document describes the semantics of the x86 auxiliary vectors.
+
+Introduction
+============
+
+ELF Auxiliary vectors enable the kernel to efficiently provide
+configuration-specific parameters to userspace. In this example, a program
+allocates an alternate stack based on the kernel-provided size::
+
+   #include <sys/auxv.h>
+   #include <elf.h>
+   #include <signal.h>
+   #include <stdlib.h>
+   #include <assert.h>
+   #include <err.h>
+
+   #ifndef AT_MINSIGSTKSZ
+   #define AT_MINSIGSTKSZ	51
+   #endif
+
+   ....
+   stack_t ss;
+
+   ss.ss_sp = malloc(ss.ss_size);
+   assert(ss.ss_sp);
+
+   ss.ss_size = getauxval(AT_MINSIGSTKSZ) + SIGSTKSZ;
+   ss.ss_flags = 0;
+
+   if (sigaltstack(&ss, NULL))
+        err(1, "sigaltstack");
+
+
+The exposed auxiliary vectors
+=============================
+
+AT_SYSINFO is used for locating the vsyscall entry point.  It is not
+exported on 64-bit mode.
+
+AT_SYSINFO_EHDR is the start address of the page containing the vDSO.
+
+AT_MINSIGSTKSZ denotes the minimum stack size required by the kernel to
+deliver a signal to user-space.  AT_MINSIGSTKSZ comprehends the space
+consumed by the kernel to accommodate the user context for the current
+hardware configuration.  It does not comprehend subsequent user-space stack
+consumption, which must be added by the user.  (e.g. Above, user-space adds
+SIGSTKSZ to AT_MINSIGSTKSZ.)
diff --git a/Documentation/x86/index.rst b/Documentation/x86/index.rst
index 4693e19..d58614d 100644
--- a/Documentation/x86/index.rst
+++ b/Documentation/x86/index.rst
@@ -35,3 +35,4 @@ x86-specific Documentation
    sva
    sgx
    features
+   elf_auxvec
diff --git a/arch/x86/include/asm/elf.h b/arch/x86/include/asm/elf.h
index 7d75008..29fea18 100644
--- a/arch/x86/include/asm/elf.h
+++ b/arch/x86/include/asm/elf.h
@@ -312,6 +312,7 @@ do {									\
 		NEW_AUX_ENT(AT_SYSINFO,	VDSO_ENTRY);			\
 		NEW_AUX_ENT(AT_SYSINFO_EHDR, VDSO_CURRENT_BASE);	\
 	}								\
+	NEW_AUX_ENT(AT_MINSIGSTKSZ, get_sigframe_size());		\
 } while (0)
 
 /*
@@ -328,6 +329,7 @@ extern unsigned long task_size_32bit(void);
 extern unsigned long task_size_64bit(int full_addr_space);
 extern unsigned long get_mmap_base(int is_legacy);
 extern bool mmap_address_hint_valid(unsigned long addr, unsigned long len);
+extern unsigned long get_sigframe_size(void);
 
 #ifdef CONFIG_X86_32
 
@@ -349,6 +351,7 @@ do {									\
 	if (vdso64_enabled)						\
 		NEW_AUX_ENT(AT_SYSINFO_EHDR,				\
 			    (unsigned long __force)current->mm->context.vdso); \
+	NEW_AUX_ENT(AT_MINSIGSTKSZ, get_sigframe_size());		\
 } while (0)
 
 /* As a historical oddity, the x32 and x86_64 vDSOs are controlled together. */
@@ -357,6 +360,7 @@ do {									\
 	if (vdso64_enabled)						\
 		NEW_AUX_ENT(AT_SYSINFO_EHDR,				\
 			    (unsigned long __force)current->mm->context.vdso); \
+	NEW_AUX_ENT(AT_MINSIGSTKSZ, get_sigframe_size());		\
 } while (0)
 
 #define AT_SYSINFO		32
diff --git a/arch/x86/include/uapi/asm/auxvec.h b/arch/x86/include/uapi/asm/auxvec.h
index 580e3c5..6beb55b 100644
--- a/arch/x86/include/uapi/asm/auxvec.h
+++ b/arch/x86/include/uapi/asm/auxvec.h
@@ -12,9 +12,9 @@
 
 /* entries in ARCH_DLINFO: */
 #if defined(CONFIG_IA32_EMULATION) || !defined(CONFIG_X86_64)
-# define AT_VECTOR_SIZE_ARCH 2
+# define AT_VECTOR_SIZE_ARCH 3
 #else /* else it's non-compat x86-64 */
-# define AT_VECTOR_SIZE_ARCH 1
+# define AT_VECTOR_SIZE_ARCH 2
 #endif
 
 #endif /* _ASM_X86_AUXVEC_H */
diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c
index 689a4b6..86e5386 100644
--- a/arch/x86/kernel/signal.c
+++ b/arch/x86/kernel/signal.c
@@ -718,6 +718,11 @@ void __init init_sigframe_size(void)
 	pr_info("max sigframe size: %lu\n", max_frame_size);
 }
 
+unsigned long get_sigframe_size(void)
+{
+	return max_frame_size;
+}
+
 static inline int is_ia32_compat_frame(struct ksignal *ksig)
 {
 	return IS_ENABLED(CONFIG_IA32_EMULATION) &&

[tip: x86/fpu] x86/signal: Detect and prevent an alternate signal stack overflow

[tip-bot2 for Chang S. Bae]

The following commit has been merged into the x86/fpu branch of tip:

Commit-ID:     2beb4a53fc3f1081cedc1c1a198c7f56cc4fc60c
Gitweb:        https://git.kernel.org/tip/2beb4a53fc3f1081cedc1c1a198c7f56cc4fc60c
Author:        Chang S. Bae <chang.seok.bae@intel.com>
AuthorDate:    Tue, 18 May 2021 13:03:19 -07:00
Committer:     Borislav Petkov <bp@suse.de>
CommitterDate: Wed, 19 May 2021 12:40:30 +02:00

x86/signal: Detect and prevent an alternate signal stack overflow

The kernel pushes context on to the userspace stack to prepare for the
user's signal handler. When the user has supplied an alternate signal
stack, via sigaltstack(2), it is easy for the kernel to verify that the
stack size is sufficient for the current hardware context.

Check if writing the hardware context to the alternate stack will exceed
it's size. If yes, then instead of corrupting user-data and proceeding with
the original signal handler, an immediate SIGSEGV signal is delivered.

Refactor the stack pointer check code from on_sig_stack() and use the new
helper.

While the kernel allows new source code to discover and use a sufficient
alternate signal stack size, this check is still necessary to protect
binaries with insufficient alternate signal stack size from data
corruption.

Fixes: c2bc11f10a39 ("x86, AVX-512: Enable AVX-512 States Context Switch")
Reported-by: Florian Weimer <fweimer@redhat.com>
Suggested-by: Jann Horn <jannh@google.com>
Suggested-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Len Brown <len.brown@intel.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20210518200320.17239-6-chang.seok.bae@intel.com
Link: https://bugzilla.kernel.org/show_bug.cgi?id=153531
---
 arch/x86/kernel/signal.c     | 24 ++++++++++++++++++++----
 include/linux/sched/signal.h | 19 ++++++++++++-------
 2 files changed, 32 insertions(+), 11 deletions(-)

diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c
index 86e5386..2ddcf21 100644
--- a/arch/x86/kernel/signal.c
+++ b/arch/x86/kernel/signal.c
@@ -239,10 +239,11 @@ get_sigframe(struct k_sigaction *ka, struct pt_regs *regs, size_t frame_size,
 	     void __user **fpstate)
 {
 	/* Default to using normal stack */
+	bool nested_altstack = on_sig_stack(regs->sp);
+	bool entering_altstack = false;
 	unsigned long math_size = 0;
 	unsigned long sp = regs->sp;
 	unsigned long buf_fx = 0;
-	int onsigstack = on_sig_stack(sp);
 	int ret;
 
 	/* redzone */
@@ -251,15 +252,23 @@ get_sigframe(struct k_sigaction *ka, struct pt_regs *regs, size_t frame_size,
 
 	/* This is the X/Open sanctioned signal stack switching.  */
 	if (ka->sa.sa_flags & SA_ONSTACK) {
-		if (sas_ss_flags(sp) == 0)
+		/*
+		 * This checks nested_altstack via sas_ss_flags(). Sensible
+		 * programs use SS_AUTODISARM, which disables that check, and
+		 * programs that don't use SS_AUTODISARM get compatible.
+		 */
+		if (sas_ss_flags(sp) == 0) {
 			sp = current->sas_ss_sp + current->sas_ss_size;
+			entering_altstack = true;
+		}
 	} else if (IS_ENABLED(CONFIG_X86_32) &&
-		   !onsigstack &&
+		   !nested_altstack &&
 		   regs->ss != __USER_DS &&
 		   !(ka->sa.sa_flags & SA_RESTORER) &&
 		   ka->sa.sa_restorer) {
 		/* This is the legacy signal stack switching. */
 		sp = (unsigned long) ka->sa.sa_restorer;
+		entering_altstack = true;
 	}
 
 	sp = fpu__alloc_mathframe(sp, IS_ENABLED(CONFIG_X86_32),
@@ -272,8 +281,15 @@ get_sigframe(struct k_sigaction *ka, struct pt_regs *regs, size_t frame_size,
 	 * If we are on the alternate signal stack and would overflow it, don't.
 	 * Return an always-bogus address instead so we will die with SIGSEGV.
 	 */
-	if (onsigstack && !likely(on_sig_stack(sp)))
+	if (unlikely((nested_altstack || entering_altstack) &&
+		     !__on_sig_stack(sp))) {
+
+		if (show_unhandled_signals && printk_ratelimit())
+			pr_info("%s[%d] overflowed sigaltstack\n",
+				current->comm, task_pid_nr(current));
+
 		return (void __user *)-1L;
+	}
 
 	/* save i387 and extended state */
 	ret = copy_fpstate_to_sigframe(*fpstate, (void __user *)buf_fx, math_size);
diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
index 3f6a0fc..ae60f83 100644
--- a/include/linux/sched/signal.h
+++ b/include/linux/sched/signal.h
@@ -537,6 +537,17 @@ static inline int kill_cad_pid(int sig, int priv)
 #define SEND_SIG_NOINFO ((struct kernel_siginfo *) 0)
 #define SEND_SIG_PRIV	((struct kernel_siginfo *) 1)
 
+static inline int __on_sig_stack(unsigned long sp)
+{
+#ifdef CONFIG_STACK_GROWSUP
+	return sp >= current->sas_ss_sp &&
+		sp - current->sas_ss_sp < current->sas_ss_size;
+#else
+	return sp > current->sas_ss_sp &&
+		sp - current->sas_ss_sp <= current->sas_ss_size;
+#endif
+}
+
 /*
  * True if we are on the alternate signal stack.
  */
@@ -554,13 +565,7 @@ static inline int on_sig_stack(unsigned long sp)
 	if (current->sas_ss_flags & SS_AUTODISARM)
 		return 0;
 
-#ifdef CONFIG_STACK_GROWSUP
-	return sp >= current->sas_ss_sp &&
-		sp - current->sas_ss_sp < current->sas_ss_size;
-#else
-	return sp > current->sas_ss_sp &&
-		sp - current->sas_ss_sp <= current->sas_ss_size;
-#endif
+	return __on_sig_stack(sp);
 }
 
 static inline int sas_ss_flags(unsigned long sp)

[tip: x86/fpu] x86/signal: Introduce helpers to get the maximum signal frame size

[tip-bot2 for Chang S. Bae]

The following commit has been merged into the x86/fpu branch of tip:

Commit-ID:     939ef713297df2cc910592305aa26af0e87f28ac
Gitweb:        https://git.kernel.org/tip/939ef713297df2cc910592305aa26af0e87f28ac
Author:        Chang S. Bae <chang.seok.bae@intel.com>
AuthorDate:    Tue, 18 May 2021 13:03:16 -07:00
Committer:     Borislav Petkov <bp@suse.de>
CommitterDate: Wed, 19 May 2021 11:46:27 +02:00

x86/signal: Introduce helpers to get the maximum signal frame size

Signal frames do not have a fixed format and can vary in size when a number
of things change: supported XSAVE features, 32 vs. 64-bit apps, etc.

Add support for a runtime method for userspace to dynamically discover
how large a signal stack needs to be.

Introduce a new variable, max_frame_size, and helper functions for the
calculation to be used in a new user interface. Set max_frame_size to a
system-wide worst-case value, instead of storing multiple app-specific
values.

Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Len Brown <len.brown@intel.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: H.J. Lu <hjl.tools@gmail.com>
Link: https://lkml.kernel.org/r/20210518200320.17239-3-chang.seok.bae@intel.com
---
 arch/x86/include/asm/fpu/signal.h |  2 +-
 arch/x86/include/asm/sigframe.h   |  2 +-
 arch/x86/kernel/cpu/common.c      |  3 ++-
 arch/x86/kernel/fpu/signal.c      | 19 ++++++++++-
 arch/x86/kernel/signal.c          | 59 ++++++++++++++++++++++++++++--
 5 files changed, 83 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/fpu/signal.h b/arch/x86/include/asm/fpu/signal.h
index 7fb516b..8b6631d 100644
--- a/arch/x86/include/asm/fpu/signal.h
+++ b/arch/x86/include/asm/fpu/signal.h
@@ -29,6 +29,8 @@ unsigned long
 fpu__alloc_mathframe(unsigned long sp, int ia32_frame,
 		     unsigned long *buf_fx, unsigned long *size);
 
+unsigned long fpu__get_fpstate_size(void);
+
 extern void fpu__init_prepare_fx_sw_frame(void);
 
 #endif /* _ASM_X86_FPU_SIGNAL_H */
diff --git a/arch/x86/include/asm/sigframe.h b/arch/x86/include/asm/sigframe.h
index 84eab27..5b1ed65 100644
--- a/arch/x86/include/asm/sigframe.h
+++ b/arch/x86/include/asm/sigframe.h
@@ -85,4 +85,6 @@ struct rt_sigframe_x32 {
 
 #endif /* CONFIG_X86_64 */
 
+void __init init_sigframe_size(void);
+
 #endif /* _ASM_X86_SIGFRAME_H */
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index a1b756c..9173dd4 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -58,6 +58,7 @@
 #include <asm/intel-family.h>
 #include <asm/cpu_device_id.h>
 #include <asm/uv/uv.h>
+#include <asm/sigframe.h>
 
 #include "cpu.h"
 
@@ -1332,6 +1333,8 @@ static void __init early_identify_cpu(struct cpuinfo_x86 *c)
 
 	fpu__init_system(c);
 
+	init_sigframe_size();
+
 #ifdef CONFIG_X86_32
 	/*
 	 * Regardless of whether PCID is enumerated, the SDM says
diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
index a4ec653..dbb304e 100644
--- a/arch/x86/kernel/fpu/signal.c
+++ b/arch/x86/kernel/fpu/signal.c
@@ -507,6 +507,25 @@ fpu__alloc_mathframe(unsigned long sp, int ia32_frame,
 
 	return sp;
 }
+
+unsigned long fpu__get_fpstate_size(void)
+{
+	unsigned long ret = xstate_sigframe_size();
+
+	/*
+	 * This space is needed on (most) 32-bit kernels, or when a 32-bit
+	 * app is running on a 64-bit kernel. To keep things simple, just
+	 * assume the worst case and always include space for 'freg_state',
+	 * even for 64-bit apps on 64-bit kernels. This wastes a bit of
+	 * space, but keeps the code simple.
+	 */
+	if ((IS_ENABLED(CONFIG_IA32_EMULATION) ||
+	     IS_ENABLED(CONFIG_X86_32)) && use_fxsr())
+		ret += sizeof(struct fregs_state);
+
+	return ret;
+}
+
 /*
  * Prepare the SW reserved portion of the fxsave memory layout, indicating
  * the presence of the extended state information in the memory layout
diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c
index a06cb10..689a4b6 100644
--- a/arch/x86/kernel/signal.c
+++ b/arch/x86/kernel/signal.c
@@ -212,6 +212,11 @@ do {									\
  * Set up a signal frame.
  */
 
+/* x86 ABI requires 16-byte alignment */
+#define FRAME_ALIGNMENT	16UL
+
+#define MAX_FRAME_PADDING	(FRAME_ALIGNMENT - 1)
+
 /*
  * Determine which stack to use..
  */
@@ -222,9 +227,9 @@ static unsigned long align_sigframe(unsigned long sp)
 	 * Align the stack pointer according to the i386 ABI,
 	 * i.e. so that on function entry ((sp + 4) & 15) == 0.
 	 */
-	sp = ((sp + 4) & -16ul) - 4;
+	sp = ((sp + 4) & -FRAME_ALIGNMENT) - 4;
 #else /* !CONFIG_X86_32 */
-	sp = round_down(sp, 16) - 8;
+	sp = round_down(sp, FRAME_ALIGNMENT) - 8;
 #endif
 	return sp;
 }
@@ -663,6 +668,56 @@ badframe:
 	return 0;
 }
 
+/*
+ * There are four different struct types for signal frame: sigframe_ia32,
+ * rt_sigframe_ia32, rt_sigframe_x32, and rt_sigframe. Use the worst case
+ * -- the largest size. It means the size for 64-bit apps is a bit more
+ * than needed, but this keeps the code simple.
+ */
+#if defined(CONFIG_X86_32) || defined(CONFIG_IA32_EMULATION)
+# define MAX_FRAME_SIGINFO_UCTXT_SIZE	sizeof(struct sigframe_ia32)
+#else
+# define MAX_FRAME_SIGINFO_UCTXT_SIZE	sizeof(struct rt_sigframe)
+#endif
+
+/*
+ * The FP state frame contains an XSAVE buffer which must be 64-byte aligned.
+ * If a signal frame starts at an unaligned address, extra space is required.
+ * This is the max alignment padding, conservatively.
+ */
+#define MAX_XSAVE_PADDING	63UL
+
+/*
+ * The frame data is composed of the following areas and laid out as:
+ *
+ * -------------------------
+ * | alignment padding     |
+ * -------------------------
+ * | (f)xsave frame        |
+ * -------------------------
+ * | fsave header          |
+ * -------------------------
+ * | alignment padding     |
+ * -------------------------
+ * | siginfo + ucontext    |
+ * -------------------------
+ */
+
+/* max_frame_size tells userspace the worst case signal stack size. */
+static unsigned long __ro_after_init max_frame_size;
+
+void __init init_sigframe_size(void)
+{
+	max_frame_size = MAX_FRAME_SIGINFO_UCTXT_SIZE + MAX_FRAME_PADDING;
+
+	max_frame_size += fpu__get_fpstate_size() + MAX_XSAVE_PADDING;
+
+	/* Userspace expects an aligned size. */
+	max_frame_size = round_up(max_frame_size, FRAME_ALIGNMENT);
+
+	pr_info("max sigframe size: %lu\n", max_frame_size);
+}
+
 static inline int is_ia32_compat_frame(struct ksignal *ksig)
 {
 	return IS_ENABLED(CONFIG_IA32_EMULATION) &&

[tip: x86/fpu] uapi/auxvec: Define the aux vector AT_MINSIGSTKSZ

[tip-bot2 for Chang S. Bae]

The following commit has been merged into the x86/fpu branch of tip:

Commit-ID:     7cd60e43a6def40ecb75deb8decc677995970d0b
Gitweb:        https://git.kernel.org/tip/7cd60e43a6def40ecb75deb8decc677995970d0b
Author:        Chang S. Bae <chang.seok.bae@intel.com>
AuthorDate:    Tue, 18 May 2021 13:03:15 -07:00
Committer:     Borislav Petkov <bp@suse.de>
CommitterDate: Wed, 19 May 2021 11:40:15 +02:00

uapi/auxvec: Define the aux vector AT_MINSIGSTKSZ

Define AT_MINSIGSTKSZ in the generic uapi header. It is already used
as generic ABI in glibc's generic elf.h, and this define will prevent
future namespace conflicts. In particular, x86 is also using this
generic definition.

Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Len Brown <len.brown@intel.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20210518200320.17239-2-chang.seok.bae@intel.com
---
 include/uapi/linux/auxvec.h | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/include/uapi/linux/auxvec.h b/include/uapi/linux/auxvec.h
index abe5f2b..c7e502b 100644
--- a/include/uapi/linux/auxvec.h
+++ b/include/uapi/linux/auxvec.h
@@ -33,5 +33,8 @@
 
 #define AT_EXECFN  31	/* filename of program */
 
+#ifndef AT_MINSIGSTKSZ
+#define AT_MINSIGSTKSZ	51	/* minimal stack size for signal delivery */
+#endif
 
 #endif /* _UAPI_LINUX_AUXVEC_H */