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authorAndy Lutomirski <luto@kernel.org>2016-02-16 15:09:03 -0800
committerIngo Molnar <mingo@kernel.org>2016-02-17 08:32:11 +0100
commit6c25da5ad55d48c41b8909bc1f4e3cd5d85bb499 (patch)
tree3dc78e9e4f4b59fa3523536e5a00f3c5e0579126 /arch/x86/kernel/signal.c
parent8ff5bd2e1e2767fbf737f84d5f92668dafe7e7b0 (diff)
x86/signal/64: Re-add support for SS in the 64-bit signal context
This is a second attempt to make the improvements from c6f2062935c8 ("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>
Diffstat (limited to 'arch/x86/kernel/signal.c')
-rw-r--r--arch/x86/kernel/signal.c63
1 files changed, 44 insertions, 19 deletions
diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c
index 52f82c7ef57d..548ddf7d6fd2 100644
--- a/arch/x86/kernel/signal.c
+++ b/arch/x86/kernel/signal.c
@@ -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))