[PATCH 3/7] syscalls/x86: use struct pt_regs based syscall calling for 64bit syscalls

Dominik Brodowski
2018-03-30T09:37:42Z

Let's make use of ARCH_HAS_SYSCALL_WRAPPER on pure 64bit x86-64 systems:

Each syscall defines a stub which takes struct pt_regs as its only
argument. It decodes just those parameters it needs, e.g:

asmlinkage long sys_xyzzy(struct pt_regs *regs)
{
return SyS_xyzzy(regs->di, regs->si, regs->dx);
}

This approach avoids leaking random user-provided register content down
the call chain.

For example, for sys_recv() which is a 4-parameter syscall, the assembly
now is (in slightly reordered fashion):

<sys_recv>:
callq <__fentry__>

/* decode regs->di, ->si, ->dx and ->r10 */
mov 0x70(%rdi),%rdi
mov 0x68(%rdi),%rsi
mov 0x60(%rdi),%rdx
mov 0x38(%rdi),%rcx

[ SyS_recv() is automatically inlined by the compiler,
as it is not [yet] used anywhere else ]
/* clear %r9 and %r8, the 5th and 6th args */
xor %r9d,%r9d
xor %r8d,%r8d

/* do the actual work */
callq __sys_recvfrom

/* cleanup and return */
cltq
retq

The only valid place in an x86-64 kernel which rightfully calls
a syscall function on its own -- vsyscall -- needs to be modified
to pass struct pt_regs onwards as well.

This patch is based on an original proof-of-concept

From: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

and was split up and heavily modified by me, in particular to base it on
ARCH_HAS_SYSCALL_WRAPPER, to limit it to 64bit-only for the time being,
and to update the vsyscall to the new calling convention.

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: x86@kernel.org
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>
---
arch/x86/Kconfig | 5 +++
arch/x86/entry/common.c | 4 ++
arch/x86/entry/syscall_64.c | 9 ++++-
arch/x86/entry/vsyscall/vsyscall_64.c | 16 ++++++++
arch/x86/include/asm/syscall.h | 4 ++
arch/x86/include/asm/syscall_wrapper.h | 69 ++++++++++++++++++++++++++++++++++
arch/x86/include/asm/syscalls.h | 7 ++++
include/linux/syscalls.h | 2 +-
8 files changed, 113 insertions(+), 3 deletions(-)
create mode 100644 arch/x86/include/asm/syscall_wrapper.h

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 0fa71a78ec99..a5db03705452 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2957,3 +2957,8 @@ source "crypto/Kconfig"
source "arch/x86/kvm/Kconfig"

source "lib/Kconfig"
+
+config SYSCALL_PTREGS
+ def_bool y
+ depends on X86_64 && !COMPAT
+ select ARCH_HAS_SYSCALL_WRAPPER
diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
index a8b066dbbf48..e1b91bffa988 100644
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -284,9 +284,13 @@ __visible void do_syscall_64(unsigned long nr, struct pt_regs *regs)
nr &= __SYSCALL_MASK;
if (likely(nr < NR_syscalls)) {
nr = array_index_nospec(nr, NR_syscalls);
+#ifdef CONFIG_SYSCALL_PTREGS
+ regs->ax = sys_call_table[nr](regs);
+#else
regs->ax = sys_call_table[nr](
regs->di, regs->si, regs->dx,
regs->r10, regs->r8, regs->r9);
+#endif
}

syscall_return_slowpath(regs);
diff --git a/arch/x86/entry/syscall_64.c b/arch/x86/entry/syscall_64.c
index c176d2fab1da..b4e724777a7d 100644
--- a/arch/x86/entry/syscall_64.c
+++ b/arch/x86/entry/syscall_64.c
@@ -7,14 +7,19 @@
#include <asm/asm-offsets.h>
#include <asm/syscall.h>

+#ifdef CONFIG_SYSCALL_PTREGS
+/* this is a lie, but it does not hurt as sys_ni_syscall just returns -EINVAL */
+extern asmlinkage long sys_ni_syscall(struct pt_regs *);
+#define __SYSCALL_64(nr, sym, qual) extern asmlinkage long sym(struct pt_regs *);
+#else /* CONFIG_SYSCALL_PTREGS */
+extern asmlinkage long sys_ni_syscall(unsigned long, unsigned long, unsigned long, unsigned long, unsigned long, unsigned long);
#define __SYSCALL_64(nr, sym, qual) extern asmlinkage long sym(unsigned long, unsigned long, unsigned long, unsigned long, unsigned long, unsigned long);
+#endif /* CONFIG_SYSCALL_PTREGS */
#include <asm/syscalls_64.h>
#undef __SYSCALL_64

#define __SYSCALL_64(nr, sym, qual) [nr] = sym,

-extern long sys_ni_syscall(unsigned long, unsigned long, unsigned long, unsigned long, unsigned long, unsigned long);
-
asmlinkage const sys_call_ptr_t sys_call_table[__NR_syscall_max+1] = {
/*
* Smells like a compiler bug -- it doesn't work
diff --git a/arch/x86/entry/vsyscall/vsyscall_64.c b/arch/x86/entry/vsyscall/vsyscall_64.c
index 8560ef68a9d6..9fad68899f82 100644
--- a/arch/x86/entry/vsyscall/vsyscall_64.c
+++ b/arch/x86/entry/vsyscall/vsyscall_64.c
@@ -227,19 +227,35 @@ bool emulate_vsyscall(struct pt_regs *regs, unsigned long address)
ret = -EFAULT;
switch (vsyscall_nr) {
case 0:
+#ifdef CONFIG_SYSCALL_PTREGS
+ /* this decodes regs->di and regs->si on its own */
+ ret = sys_gettimeofday(regs);
+#else
ret = sys_gettimeofday(
(struct timeval __user *)regs->di,
(struct timezone __user *)regs->si);
+#endif /* CONFIG_SYSCALL_PTREGS */
break;

case 1:
+#ifdef CONFIG_SYSCALL_PTREGS
+ /* this decodes regs->di on its own */
+ ret = sys_time(regs);
+#else
ret = sys_time((time_t __user *)regs->di);
+#endif /* CONFIG_SYSCALL_PTREGS */
break;

case 2:
+#ifdef CONFIG_SYSCALL_PTREGS
+ /* this decodes regs->di, regs->si and regs->dx on its own */
+ regs->dx = 0;
+ ret = sys_getcpu(regs);
+#else
ret = sys_getcpu((unsigned __user *)regs->di,
(unsigned __user *)regs->si,
NULL);
+#endif /* CONFIG_SYSCALL_PTREGS */
break;
}

diff --git a/arch/x86/include/asm/syscall.h b/arch/x86/include/asm/syscall.h
index 03eedc21246d..8702c7951bc7 100644
--- a/arch/x86/include/asm/syscall.h
+++ b/arch/x86/include/asm/syscall.h
@@ -20,9 +20,13 @@
#include <asm/thread_info.h> /* for TS_COMPAT */
#include <asm/unistd.h>

+#ifdef CONFIG_SYSCALL_PTREGS
+typedef asmlinkage long (*sys_call_ptr_t)(struct pt_regs *);
+#else
typedef asmlinkage long (*sys_call_ptr_t)(unsigned long, unsigned long,
unsigned long, unsigned long,
unsigned long, unsigned long);
+#endif /* CONFIG_SYSCALL_PTREGS */
extern const sys_call_ptr_t sys_call_table[];

#if defined(CONFIG_X86_32)
diff --git a/arch/x86/include/asm/syscall_wrapper.h b/arch/x86/include/asm/syscall_wrapper.h
new file mode 100644
index 000000000000..ca928adf4a53
--- /dev/null
+++ b/arch/x86/include/asm/syscall_wrapper.h
@@ -0,0 +1,69 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * syscall_wrapper.h - x86 specific wrappers to syscall definitions
+ */
+
+#ifndef _ASM_X86_SYSCALL_WRAPPER_H
+#define _ASM_X86_SYSCALL_WRAPPER_H
+
+/*
+ * Instead of the generic __SYSCALL_DEFINEx() definition, this macro takes
+ * struct pt_regs *regs as the only argument of the syscall stub named
+ * sys_*(). It decodes just the registers it needs and passes them on to
+ * the SyS_*() wrapper and then to the SYSC_*() function doing the actual job.
+ * These wrappers and functions are inlined, meaning that the assembly looks
+ * as follows (slightly re-ordered):
+ *
+ * <sys_recv>: <-- syscall with 4 parameters
+ * callq <__fentry__>
+ *
+ * mov 0x70(%rdi),%rdi <-- decode regs->di
+ * mov 0x68(%rdi),%rsi <-- decode regs->si
+ * mov 0x60(%rdi),%rdx <-- decode regs->dx
+ * mov 0x38(%rdi),%rcx <-- decode regs->r10
+ *
+ * xor %r9d,%r9d <-- clear %r9
+ * xor %r8d,%r8d <-- clear %r8
+ *
+ * callq __sys_recvfrom <-- do the actual work in __sys_recvfrom()
+ * which takes 6 arguments
+ *
+ * cltq <-- extend return value to 64bit
+ * retq <-- return
+ *
+ * This approach avoids leaking random user-provided register content down
+ * the call chain.
+ *
+ * As the generic SYSCALL_DEFINE0() macro does not decode any parameters for
+ * obvious reasons, there is no need to override it.
+ */
+#define __SYSCALL_DEFINEx(x, name, ...) \
+ asmlinkage long sys##name(struct pt_regs *regs); \
+ ALLOW_ERROR_INJECTION(sys##name, ERRNO); \
+ static long SyS##name(__MAP(x,__SC_LONG,__VA_ARGS__)); \
+ static inline long SYSC##name(__MAP(x,__SC_DECL,__VA_ARGS__)); \
+ asmlinkage long sys##name(struct pt_regs *regs) \
+ { \
+ return SyS##name(__MAP(x,__SC_ARGS \
+ ,,regs->di,,regs->si,,regs->dx \
+ ,,regs->r10,,regs->r8,,regs->r9)); \
+ } \
+ static long SyS##name(__MAP(x,__SC_LONG,__VA_ARGS__)) \
+ { \
+ long ret = SYSC##name(__MAP(x,__SC_CAST,__VA_ARGS__)); \
+ __MAP(x,__SC_TEST,__VA_ARGS__); \
+ __PROTECT(x, ret,__MAP(x,__SC_ARGS,__VA_ARGS__)); \
+ return ret; \
+ } \
+ static inline long SYSC##name(__MAP(x,__SC_DECL,__VA_ARGS__))
+
+/*
+ * for VSYSCALLS, we need to declare these three syscalls with the new
+ * pt_regs-based calling convention for in-kernel use.
+ */
+struct pt_regs;
+asmlinkage long sys_getcpu(struct pt_regs *regs); /* di, si, dx */
+asmlinkage long sys_gettimeofday(struct pt_regs *regs); /* di, si */
+asmlinkage long sys_time(struct pt_regs *regs); /* di */
+
+#endif /* _ASM_X86_SYSCALL_WRAPPER_H */
diff --git a/arch/x86/include/asm/syscalls.h b/arch/x86/include/asm/syscalls.h
index ae6e05fdc24b..e4ad93c05f02 100644
--- a/arch/x86/include/asm/syscalls.h
+++ b/arch/x86/include/asm/syscalls.h
@@ -18,6 +18,12 @@
/* Common in X86_32 and X86_64 */
/* kernel/ioport.c */
long ksys_ioperm(unsigned long from, unsigned long num, int turn_on);
+
+#ifndef CONFIG_SYSCALL_PTREGS
+/*
+ * If CONFIG_SYSCALL_PTREGS is enabled, a different syscall calling convention
+ * is used. Do not include these -- invalid -- prototypes then
+ */
asmlinkage long sys_ioperm(unsigned long, unsigned long, int);
asmlinkage long sys_iopl(unsigned int);

@@ -53,4 +59,5 @@ asmlinkage long sys_mmap(unsigned long, unsigned long, unsigned long,
unsigned long, unsigned long, unsigned long);

#endif /* CONFIG_X86_32 */
+#endif /* CONFIG_SYSCALL_PTREGS */
#endif /* _ASM_X86_SYSCALLS_H */
diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index 503ab245d4ce..d7168b3a4b4c 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -102,7 +102,7 @@ union bpf_attr;
* for SYSCALL_DEFINE<n>/COMPAT_SYSCALL_DEFINE<n>
*/
#define __MAP0(m,...)
-#define __MAP1(m,t,a) m(t,a)
+#define __MAP1(m,t,a,...) m(t,a)
#define __MAP2(m,t,a,...) m(t,a), __MAP1(m,__VA_ARGS__)
#define __MAP3(m,t,a,...) m(t,a), __MAP2(m,__VA_ARGS__)
#define __MAP4(m,t,a,...) m(t,a), __MAP3(m,__VA_ARGS__)
--
2.16.3