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-rw-r--r--arch/x86/entry/common.c111
1 files changed, 77 insertions, 34 deletions
diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
index 9c0b26ae51..6356060caa 100644
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -19,6 +19,7 @@
#include <linux/nospec.h>
#include <linux/syscalls.h>
#include <linux/uaccess.h>
+#include <linux/init.h>
#ifdef CONFIG_XEN_PV
#include <xen/xen-ops.h>
@@ -71,7 +72,8 @@ static __always_inline bool do_syscall_x32(struct pt_regs *regs, int nr)
return false;
}
-__visible noinstr void do_syscall_64(struct pt_regs *regs, int nr)
+/* Returns true to return using SYSRET, or false to use IRET */
+__visible noinstr bool do_syscall_64(struct pt_regs *regs, int nr)
{
add_random_kstack_offset();
nr = syscall_enter_from_user_mode(regs, nr);
@@ -85,6 +87,46 @@ __visible noinstr void do_syscall_64(struct pt_regs *regs, int nr)
instrumentation_end();
syscall_exit_to_user_mode(regs);
+
+ /*
+ * Check that the register state is valid for using SYSRET to exit
+ * to userspace. Otherwise use the slower but fully capable IRET
+ * exit path.
+ */
+
+ /* XEN PV guests always use the IRET path */
+ if (cpu_feature_enabled(X86_FEATURE_XENPV))
+ return false;
+
+ /* SYSRET requires RCX == RIP and R11 == EFLAGS */
+ if (unlikely(regs->cx != regs->ip || regs->r11 != regs->flags))
+ return false;
+
+ /* CS and SS must match the values set in MSR_STAR */
+ if (unlikely(regs->cs != __USER_CS || regs->ss != __USER_DS))
+ return false;
+
+ /*
+ * On Intel CPUs, SYSRET with non-canonical RCX/RIP will #GP
+ * in kernel space. This essentially lets the user take over
+ * the kernel, since userspace controls RSP.
+ *
+ * TASK_SIZE_MAX covers all user-accessible addresses other than
+ * the deprecated vsyscall page.
+ */
+ if (unlikely(regs->ip >= TASK_SIZE_MAX))
+ return false;
+
+ /*
+ * SYSRET cannot restore RF. It can restore TF, but unlike IRET,
+ * restoring TF results in a trap from userspace immediately after
+ * SYSRET.
+ */
+ if (unlikely(regs->flags & (X86_EFLAGS_RF | X86_EFLAGS_TF)))
+ return false;
+
+ /* Use SYSRET to exit to userspace */
+ return true;
}
#endif
@@ -98,7 +140,13 @@ static __always_inline int syscall_32_enter(struct pt_regs *regs)
}
#ifdef CONFIG_IA32_EMULATION
-bool __ia32_enabled __ro_after_init = true;
+bool __ia32_enabled __ro_after_init = !IS_ENABLED(CONFIG_IA32_EMULATION_DEFAULT_DISABLED);
+
+static int ia32_emulation_override_cmdline(char *arg)
+{
+ return kstrtobool(arg, &__ia32_enabled);
+}
+early_param("ia32_emulation", ia32_emulation_override_cmdline);
#endif
/*
@@ -277,8 +325,8 @@ static noinstr bool __do_fast_syscall_32(struct pt_regs *regs)
return true;
}
-/* Returns 0 to return using IRET or 1 to return using SYSEXIT/SYSRETL. */
-__visible noinstr long do_fast_syscall_32(struct pt_regs *regs)
+/* Returns true to return using SYSEXIT/SYSRETL, or false to use IRET */
+__visible noinstr bool do_fast_syscall_32(struct pt_regs *regs)
{
/*
* Called using the internal vDSO SYSENTER/SYSCALL32 calling
@@ -296,41 +344,36 @@ __visible noinstr long do_fast_syscall_32(struct pt_regs *regs)
/* Invoke the syscall. If it failed, keep it simple: use IRET. */
if (!__do_fast_syscall_32(regs))
- return 0;
+ return false;
-#ifdef CONFIG_X86_64
/*
- * Opportunistic SYSRETL: if possible, try to return using SYSRETL.
- * SYSRETL is available on all 64-bit CPUs, so we don't need to
- * bother with SYSEXIT.
- *
- * Unlike 64-bit opportunistic SYSRET, we can't check that CX == IP,
- * because the ECX fixup above will ensure that this is essentially
- * never the case.
+ * Check that the register state is valid for using SYSRETL/SYSEXIT
+ * to exit to userspace. Otherwise use the slower but fully capable
+ * IRET exit path.
*/
- return regs->cs == __USER32_CS && regs->ss == __USER_DS &&
- regs->ip == landing_pad &&
- (regs->flags & (X86_EFLAGS_RF | X86_EFLAGS_TF)) == 0;
-#else
- /*
- * Opportunistic SYSEXIT: if possible, try to return using SYSEXIT.
- *
- * Unlike 64-bit opportunistic SYSRET, we can't check that CX == IP,
- * because the ECX fixup above will ensure that this is essentially
- * never the case.
- *
- * We don't allow syscalls at all from VM86 mode, but we still
- * need to check VM, because we might be returning from sys_vm86.
- */
- return static_cpu_has(X86_FEATURE_SEP) &&
- regs->cs == __USER_CS && regs->ss == __USER_DS &&
- regs->ip == landing_pad &&
- (regs->flags & (X86_EFLAGS_RF | X86_EFLAGS_TF | X86_EFLAGS_VM)) == 0;
-#endif
+
+ /* XEN PV guests always use the IRET path */
+ if (cpu_feature_enabled(X86_FEATURE_XENPV))
+ return false;
+
+ /* EIP must point to the VDSO landing pad */
+ if (unlikely(regs->ip != landing_pad))
+ return false;
+
+ /* CS and SS must match the values set in MSR_STAR */
+ if (unlikely(regs->cs != __USER32_CS || regs->ss != __USER_DS))
+ return false;
+
+ /* If the TF, RF, or VM flags are set, use IRET */
+ if (unlikely(regs->flags & (X86_EFLAGS_RF | X86_EFLAGS_TF | X86_EFLAGS_VM)))
+ return false;
+
+ /* Use SYSRETL/SYSEXIT to exit to userspace */
+ return true;
}
-/* Returns 0 to return using IRET or 1 to return using SYSEXIT/SYSRETL. */
-__visible noinstr long do_SYSENTER_32(struct pt_regs *regs)
+/* Returns true to return using SYSEXIT/SYSRETL, or false to use IRET */
+__visible noinstr bool do_SYSENTER_32(struct pt_regs *regs)
{
/* SYSENTER loses RSP, but the vDSO saved it in RBP. */
regs->sp = regs->bp;