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x86/signal/64: Re-add support for SS in the 64-bit signal context 

This is a second attempt to make the improvements from c6f2062935
("x86/signal/64: Fix SS handling for signals delivered to 64-bit
programs"), which was reverted by 51adbfbba5c6 ("x86/signal/64: Add
support for SS in the 64-bit signal context").

This adds two new uc_flags flags.  UC_SIGCONTEXT_SS will be set for
all 64-bit signals (including x32).  It indicates that the saved SS
field is valid and that the kernel supports the new behavior.

The goal is to fix a problems with signal handling in 64-bit tasks:
SS wasn't saved in the 64-bit signal context, making it awkward to
determine what SS was at the time of signal delivery and making it
impossible to return to a non-flat SS (as calling sigreturn clobbers
SS).

This also made it extremely difficult for 64-bit tasks to return to
fully-defined 16-bit contexts, because only the kernel can easily do
espfix64, but sigreturn was unable to set a non-flag SS:ESP.
(DOSEMU has a monstrous hack to partially work around this
limitation.)

If we could go back in time, the correct fix would be to make 64-bit
signals work just like 32-bit signals with respect to SS: save it
in signal context, reset it when delivering a signal, and restore
it in sigreturn.

Unfortunately, doing that (as I tried originally) breaks DOSEMU:
DOSEMU wouldn't reset the signal context's SS when clearing the LDT
and changing the saved CS to 64-bit mode, since it predates the SS
context field existing in the first place.

This patch is a bit more complicated, and it tries to balance a
bunch of goals.  It makes most cases of changing ucontext->ss during
signal handling work as expected.

I do this by special-casing the interesting case.  On sigreturn,
ucontext->ss will be honored by default, unless the ucontext was
created from scratch by an old program and had a 64-bit CS
(unfortunately, CRIU can do this) or was the result of changing a
32-bit signal context to 64-bit without resetting SS (as DOSEMU
does).

For the benefit of new 64-bit software that uses segmentation (new
versions of DOSEMU might), the new behavior can be detected with a
new ucontext flag UC_SIGCONTEXT_SS.

To avoid compilation issues, __pad0 is left as an alias for ss in
ucontext.

The nitty-gritty details are documented in the header file.

This patch also re-enables the sigreturn_64 and ldt_gdt_64 selftests,
as the kernel change allows both of them to pass.

Tested-by: Stas Sergeev <stsp@list.ru>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Acked-by: Borislav Petkov <bp@alien8.de>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Cyrill Gorcunov <gorcunov@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Pavel Emelyanov <xemul@parallels.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/749149cbfc3e75cd7fcdad69a854b399d792cc6f.1455664054.git.luto@kernel.org
[ Small readability edit. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Andy Lutomirski 2016-02-16 15:09:03 -08:00 committed by Ingo Molnar
parent 8ff5bd2e1e
commit 6c25da5ad5
5 changed files with 99 additions and 30 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

1
arch/x86/include/asm/sighandling.h
View File

@ -13,7 +13,6 @@
X86_EFLAGS_CF | X86_EFLAGS_RF)
void signal_fault(struct pt_regs *regs, void __user *frame, char *where);
int restore_sigcontext(struct pt_regs *regs, struct sigcontext __user *sc);
int setup_sigcontext(struct sigcontext __user *sc, void __user *fpstate,
struct pt_regs *regs, unsigned long mask);

7
arch/x86/include/uapi/asm/sigcontext.h
View File

@ -256,7 +256,7 @@ struct sigcontext_64 {
__u16 cs;
__u16 gs;
__u16 fs;
__u16 __pad0;
__u16 ss;
__u64 err;
__u64 trapno;
__u64 oldmask;
@ -368,7 +368,10 @@ struct sigcontext {
*/
__u16 gs;
__u16 fs;
__u16 __pad0;
union {
__u16 ss; /* If UC_SIGCONTEXT_SS */
__u16 __pad0; /* Alias name for old (!UC_SIGCONTEXT_SS) user-space */
};
__u64 err;
__u64 trapno;
__u64 oldmask;

53
arch/x86/include/uapi/asm/ucontext.h
View File

@ -1,11 +1,54 @@
#ifndef _ASM_X86_UCONTEXT_H
#define _ASM_X86_UCONTEXT_H
#define UC_FP_XSTATE 0x1 /* indicates the presence of extended state
* information in the memory layout pointed
* by the fpstate pointer in the ucontext's
* sigcontext struct (uc_mcontext).
*/
/*
* Indicates the presence of extended state information in the memory
* layout pointed by the fpstate pointer in the ucontext's sigcontext
* struct (uc_mcontext).
*/
#define UC_FP_XSTATE 0x1
#ifdef __x86_64__
/*
* UC_SIGCONTEXT_SS will be set when delivering 64-bit or x32 signals on
* kernels that save SS in the sigcontext. All kernels that set
* UC_SIGCONTEXT_SS will correctly restore at least the low 32 bits of esp
* regardless of SS (i.e. they implement espfix).
*
* Kernels that set UC_SIGCONTEXT_SS will also set UC_STRICT_RESTORE_SS
* when delivering a signal that came from 64-bit code.
*
* Sigreturn restores SS as follows:
*
* if (saved SS is valid || UC_STRICT_RESTORE_SS is set ||
* saved CS is not 64-bit)
* new SS = saved SS (will fail IRET and signal if invalid)
* else
* new SS = a flat 32-bit data segment
*
* This behavior serves three purposes:
*
* - Legacy programs that construct a 64-bit sigcontext from scratch
* with zero or garbage in the SS slot (e.g. old CRIU) and call
* sigreturn will still work.
*
* - Old DOSEMU versions sometimes catch a signal from a segmented
* context, delete the old SS segment (with modify_ldt), and change
* the saved CS to a 64-bit segment. These DOSEMU versions expect
* sigreturn to send them back to 64-bit mode without killing them,
* despite the fact that the SS selector when the signal was raised is
* no longer valid. UC_STRICT_RESTORE_SS will be clear, so the kernel
* will fix up SS for these DOSEMU versions.
*
* - Old and new programs that catch a signal and return without
* modifying the saved context will end up in exactly the state they
* started in, even if they were running in a segmented context when
* the signal was raised.. Old kernels would lose track of the
* previous SS value.
*/
#define UC_SIGCONTEXT_SS 0x2
#define UC_STRICT_RESTORE_SS 0x4
#endif
#include <asm-generic/ucontext.h>

63
arch/x86/kernel/signal.c
View File

@ -90,7 +90,9 @@ static void force_valid_ss(struct pt_regs *regs)
}
#endif
int restore_sigcontext(struct pt_regs *regs, struct sigcontext __user *sc)
static int restore_sigcontext(struct pt_regs *regs,
struct sigcontext __user *sc,
unsigned long uc_flags)
{
unsigned long buf_val;
void __user *buf;
@ -123,15 +125,18 @@ int restore_sigcontext(struct pt_regs *regs, struct sigcontext __user *sc)
COPY(r15);
#endif /* CONFIG_X86_64 */
#ifdef CONFIG_X86_32
COPY_SEG_CPL3(cs);
COPY_SEG_CPL3(ss);
#else /* !CONFIG_X86_32 */
/* Kernel saves and restores only the CS segment register on signals,
* which is the bare minimum needed to allow mixed 32/64-bit code.
* App's signal handler can save/restore other segments if needed. */
COPY_SEG_CPL3(cs);
#endif /* CONFIG_X86_32 */
#ifdef CONFIG_X86_64
/*
* Fix up SS if needed for the benefit of old DOSEMU and
* CRIU.
*/
if (unlikely(!(uc_flags & UC_STRICT_RESTORE_SS) &&
user_64bit_mode(regs)))
force_valid_ss(regs);
#endif
get_user_ex(tmpflags, &sc->flags);
regs->flags = (regs->flags & ~FIX_EFLAGS) | (tmpflags & FIX_EFLAGS);
@ -194,6 +199,7 @@ int setup_sigcontext(struct sigcontext __user *sc, void __user *fpstate,
put_user_ex(regs->cs, &sc->cs);
put_user_ex(0, &sc->gs);
put_user_ex(0, &sc->fs);
put_user_ex(regs->ss, &sc->ss);
#endif /* CONFIG_X86_32 */
put_user_ex(fpstate, &sc->fpstate);
@ -432,6 +438,21 @@ static int __setup_rt_frame(int sig, struct ksignal *ksig,
return 0;
}
#else /* !CONFIG_X86_32 */
static unsigned long frame_uc_flags(struct pt_regs *regs)
{
unsigned long flags;
if (cpu_has_xsave)
flags = UC_FP_XSTATE | UC_SIGCONTEXT_SS;
else
flags = UC_SIGCONTEXT_SS;
if (likely(user_64bit_mode(regs)))
flags |= UC_STRICT_RESTORE_SS;
return flags;
}
static int __setup_rt_frame(int sig, struct ksignal *ksig,
sigset_t *set, struct pt_regs *regs)
{
@ -451,10 +472,7 @@ static int __setup_rt_frame(int sig, struct ksignal *ksig,
put_user_try {
/* Create the ucontext. */
if (cpu_has_xsave)
put_user_ex(UC_FP_XSTATE, &frame->uc.uc_flags);
else
put_user_ex(0, &frame->uc.uc_flags);
put_user_ex(frame_uc_flags(regs), &frame->uc.uc_flags);
put_user_ex(0, &frame->uc.uc_link);
save_altstack_ex(&frame->uc.uc_stack, regs->sp);
@ -536,10 +554,7 @@ static int x32_setup_rt_frame(struct ksignal *ksig,
put_user_try {
/* Create the ucontext. */
if (cpu_has_xsave)
put_user_ex(UC_FP_XSTATE, &frame->uc.uc_flags);
else
put_user_ex(0, &frame->uc.uc_flags);
put_user_ex(frame_uc_flags(regs), &frame->uc.uc_flags);
put_user_ex(0, &frame->uc.uc_link);
compat_save_altstack_ex(&frame->uc.uc_stack, regs->sp);
put_user_ex(0, &frame->uc.uc__pad0);
@ -601,7 +616,11 @@ asmlinkage unsigned long sys_sigreturn(void)
set_current_blocked(&set);
if (restore_sigcontext(regs, &frame->sc))
/*
* x86_32 has no uc_flags bits relevant to restore_sigcontext.
* Save a few cycles by skipping the __get_user.
*/
if (restore_sigcontext(regs, &frame->sc, 0))
goto badframe;
return regs->ax;
@ -617,16 +636,19 @@ asmlinkage long sys_rt_sigreturn(void)
struct pt_regs *regs = current_pt_regs();
struct rt_sigframe __user *frame;
sigset_t set;
unsigned long uc_flags;
frame = (struct rt_sigframe __user *)(regs->sp - sizeof(long));
if (!access_ok(VERIFY_READ, frame, sizeof(*frame)))
goto badframe;
if (__copy_from_user(&set, &frame->uc.uc_sigmask, sizeof(set)))
goto badframe;
if (__get_user(uc_flags, &frame->uc.uc_flags))
goto badframe;
set_current_blocked(&set);
if (restore_sigcontext(regs, &frame->uc.uc_mcontext))
if (restore_sigcontext(regs, &frame->uc.uc_mcontext, uc_flags))
goto badframe;
if (restore_altstack(&frame->uc.uc_stack))
@ -813,6 +835,7 @@ asmlinkage long sys32_x32_rt_sigreturn(void)
struct pt_regs *regs = current_pt_regs();
struct rt_sigframe_x32 __user *frame;
sigset_t set;
unsigned long uc_flags;
frame = (struct rt_sigframe_x32 __user *)(regs->sp - 8);
@ -820,10 +843,12 @@ asmlinkage long sys32_x32_rt_sigreturn(void)
goto badframe;
if (__copy_from_user(&set, &frame->uc.uc_sigmask, sizeof(set)))
goto badframe;
if (__get_user(uc_flags, &frame->uc.uc_flags))
goto badframe;
set_current_blocked(&set);
if (restore_sigcontext(regs, &frame->uc.uc_mcontext))
if (restore_sigcontext(regs, &frame->uc.uc_mcontext, uc_flags))
goto badframe;
if (compat_restore_altstack(&frame->uc.uc_stack))

5
tools/testing/selftests/x86/Makefile
View File

@ -5,10 +5,9 @@ include ../lib.mk
.PHONY: all all_32 all_64 warn_32bit_failure clean
TARGETS_C_BOTHBITS := single_step_syscall sysret_ss_attrs syscall_nt ptrace_syscall \
check_initial_reg_state
TARGETS_C_32BIT_ONLY := entry_from_vm86 syscall_arg_fault sigreturn test_syscall_vdso unwind_vdso \
check_initial_reg_state sigreturn ldt_gdt
TARGETS_C_32BIT_ONLY := entry_from_vm86 syscall_arg_fault test_syscall_vdso unwind_vdso \
test_FCMOV test_FCOMI test_FISTTP \
ldt_gdt \
vdso_restorer
TARGETS_C_32BIT_ALL := $(TARGETS_C_BOTHBITS) $(TARGETS_C_32BIT_ONLY)