x86/elf: Support a new ELF aux vector AT_MINSIGSTKSZ
authorChang S. Bae <chang.seok.bae@intel.com>
Tue, 18 May 2021 20:03:17 +0000 (13:03 -0700)
committerBorislav Petkov <bp@suse.de>
Wed, 19 May 2021 10:18:45 +0000 (12:18 +0200)
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 [new file with mode: 0644]
Documentation/x86/index.rst
arch/x86/include/asm/elf.h
arch/x86/include/uapi/asm/auxvec.h
arch/x86/kernel/signal.c

diff --git a/Documentation/x86/elf_auxvec.rst b/Documentation/x86/elf_auxvec.rst
new file mode 100644 (file)
index 0000000..18e4744
--- /dev/null
@@ -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.)
index 4693e192b447cc3a96fa114504bb8c18d29cc09f..d58614d5cde6b88ca267a8323b8fb2868f978e19 100644 (file)
@@ -35,3 +35,4 @@ x86-specific Documentation
    sva
    sgx
    features
+   elf_auxvec
index 7d7500806af8a900694e5a5fe5b8440b70a5c522..29fea180a6658e84bd0a094d4e767558cc91b4e1 100644 (file)
@@ -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
index 580e3c567046703c5e8752512a52d9e6ab88106b..6beb55bbefa40b475fea660075be2836b1771246 100644 (file)
@@ -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 */
index 689a4b6dd18fc4cfbda3f62d59a36c092e80c7fc..86e53868159ad6f3ee45f28a9ae60f377a2e9bd3 100644 (file)
@@ -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) &&