x86: Make a space in jmpbuf for shadow stack pointer

System Internals / glibc - H.J. Lu [gmail.com] - 30 November 2017 12:58 EST

To support Shadow Stack (SHSTK) in Intel Control-flow Enforcement Technology (CET) in setjmp/longjmp, we need to save shadow stack pointer in jmp_buf. The __saved_mask field in jmp_buf has type of __sigset_t. On Linux, __sigset_t is defined as

#define _SIGSET_NWORDS (1024 / (8 * sizeof (unsigned long int))) typedef struct { unsigned long int __val[_SIGSET_NWORDS]; } __sigset_t;

which is much bigger than expected by the __sigprocmask system call, which has

typedef struct { unsigned long sig[_NSIG_WORDS]; } sigset_t;

For Linux/x86, we can shrink __sigset_t used by __saved_mask in jmp_buf to add paddings for shadow stack pointer. As long as the new __sigset_t is not smaller than sigset_t expected by the __sigprocmask system call, it should work correctly.

This patch adds an internal header file, , to define __jmp_buf_sigset_t for __saved_mask in jmp_buf for Linux/x86 with a space to store shadow stack pointer. It verifies __jmp_buf_sigset_t has the suitable size for the __sigprocmask system call. A run-time test, tst-saved_mask-1.c, is added to verify that size of __jmp_buf_sigset_t is sufficient. If its size is too small, the test fails with

rt_sigprocmask(SIG_SETMASK, strace: umoven: short read (4 < 8) @0x7fa8aa28effc 0x7fa8aa28effc, NULL, 8) = -1 EFAULT (Bad address) rt_sigprocmask(SIG_SETMASK, strace: umoven: short read (4 < 8) @0x7fa8aa28effc 0x7fa8aa28effc, NULL, 8) = -1 EFAULT (Bad address) rt_sigprocmask(SIG_SETMASK, NULL, 0x7fa8aa28effc, 8) = -1 EFAULT (Bad address) exit_group(1) = ?

Tested with build-many-glibcs.py.

- debug/longjmp_chk.c: Include instead of .
- setjmp/longjmp.c: Include instead of . (__libc_siglongjmp): Cast &env[0].__saved_mask to "sigset_t *".
- setjmp/sigjmp.c: Include instead of . (__sigjmp_save): Cast &env[0].__saved_mask to "sigset_t *".
- sysdeps/generic/setjmpP.h: New file.
- sysdeps/unix/sysv/linux/x86/jmp_buf-ssp.sym: Likewise.
- sysdeps/unix/sysv/linux/x86/setjmpP.h: Likewise.
- sysdeps/unix/sysv/linux/x86/tst-saved_mask-1.c: Likewise.
- sysdeps/unix/sysv/linux/x86/Makefile (gen-as-const-headers): Add jmp_buf-ssp.sym. (tests): Add tst-saved_mask-1.

f33632ccd1 x86: Make a space in jmpbuf for shadow stack pointer
ChangeLog | 16 ++++++
debug/longjmp_chk.c | 2 +-
setjmp/longjmp.c | 5 +-
setjmp/sigjmp.c | 4 +-
sysdeps/generic/setjmpP.h | 24 +++++++++
sysdeps/unix/sysv/linux/x86/Makefile | 5 ++
sysdeps/unix/sysv/linux/x86/jmp_buf-ssp.sym | 5 ++
sysdeps/unix/sysv/linux/x86/setjmpP.h | 67 ++++++++++++++++++++++++++
sysdeps/unix/sysv/linux/x86/tst-saved_mask-1.c | 55 +++++++++++++++++++++
9 files changed, 178 insertions(+), 5 deletions(-)

Upstream: sourceware.org


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