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Subject[tip:x86/vdso] x86-64: Clean up vsyscall emulation and remove fixed-address ret
Commit-ID:  7dc0452808b75615d1dc4d5160b26aaabc6a7300
Gitweb: http://git.kernel.org/tip/7dc0452808b75615d1dc4d5160b26aaabc6a7300
Author: Andy Lutomirski <luto@MIT.EDU>
AuthorDate: Mon, 6 Jun 2011 13:27:24 -0400
Committer: Ingo Molnar <mingo@elte.hu>
CommitDate: Mon, 6 Jun 2011 19:47:20 +0200

x86-64: Clean up vsyscall emulation and remove fixed-address ret

Remove the fixed-address ret in the vsyscall emulation stubs.
As a result, int 0xcc no longer works if executed from user
address space (which is OK -- nothing sensible should do that).

Removing support for int 0xcc in user address space cleans up
the code considerably.

Signed-off-by: Andy Lutomirski <luto@mit.edu>
Cc: pageexec@freemail.hu
Cc: Mikael Pettersson <mikpe@it.uu.se>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/bd468a0e02befab146667fab5de57df7332d54d9.1307380481.git.luto@mit.edu
Signed-off-by: Ingo Molnar <mingo@elte.hu>
---
arch/x86/include/asm/vsyscall.h | 10 ++++-
arch/x86/kernel/vsyscall_64.c | 78 +++++++++++--------------------------
arch/x86/kernel/vsyscall_emu_64.S | 17 +--------
3 files changed, 32 insertions(+), 73 deletions(-)
diff --git a/arch/x86/include/asm/vsyscall.h b/arch/x86/include/asm/vsyscall.h
index 293ae08..bb710cb 100644
--- a/arch/x86/include/asm/vsyscall.h
+++ b/arch/x86/include/asm/vsyscall.h
@@ -32,9 +32,15 @@ extern struct timezone sys_tz;
extern void map_vsyscall(void);

/* Emulation */
-static inline bool in_vsyscall_page(unsigned long addr)
+
+static inline bool is_vsyscall_entry(unsigned long addr)
+{
+ return (addr & ~0xC00UL) == VSYSCALL_START;
+}
+
+static inline int vsyscall_entry_nr(unsigned long addr)
{
- return (addr & ~(PAGE_SIZE - 1)) == VSYSCALL_START;
+ return (addr & 0xC00UL) >> 10;
}

#endif /* __KERNEL__ */
diff --git a/arch/x86/kernel/vsyscall_64.c b/arch/x86/kernel/vsyscall_64.c
index 0b50b3c..04540f7 100644
--- a/arch/x86/kernel/vsyscall_64.c
+++ b/arch/x86/kernel/vsyscall_64.c
@@ -96,25 +96,10 @@ static void warn_bad_vsyscall(const char *level, struct pt_regs *regs,

tsk = current;

- printk("%s%s[%d] %s ip:%lx sp:%lx ax:%lx si:%lx di:%lx",
+ printk("%s%s[%d] %s ip:%lx sp:%lx ax:%lx si:%lx di:%lx\n",
level, tsk->comm, task_pid_nr(tsk),
message,
regs->ip - 2, regs->sp, regs->ax, regs->si, regs->di);
- if (!in_vsyscall_page(regs->ip - 2))
- print_vma_addr(" in ", regs->ip - 2);
- printk("\n");
-}
-
-/* al values for each vsyscall; see vsyscall_emu_64.S for why. */
-static u8 vsyscall_nr_to_al[] = {0xcc, 0xce, 0xf0};
-
-static int al_to_vsyscall_nr(u8 al)
-{
- int i;
- for (i = 0; i < ARRAY_SIZE(vsyscall_nr_to_al); i++)
- if (vsyscall_nr_to_al[i] == al)
- return i;
- return -1;
}

#ifdef CONFIG_COMPAT_VSYSCALLS
@@ -141,17 +126,12 @@ vtime(time_t *t)

#endif /* CONFIG_COMPAT_VSYSCALLS */

-/* If CONFIG_COMPAT_VSYSCALLS=y, then this is incorrect for vsyscall_nr == 1. */
-static inline unsigned long vsyscall_intcc_addr(int vsyscall_nr)
-{
- return VSYSCALL_START + 1024*vsyscall_nr + 2;
-}
-
void dotraplinkage do_emulate_vsyscall(struct pt_regs *regs, long error_code)
{
static DEFINE_RATELIMIT_STATE(rs, 3600 * HZ, 3);
struct task_struct *tsk;
const char *vsyscall_name;
+ unsigned long caller;
int vsyscall_nr;
long ret;

@@ -160,39 +140,26 @@ void dotraplinkage do_emulate_vsyscall(struct pt_regs *regs, long error_code)

local_irq_enable();

- vsyscall_nr = al_to_vsyscall_nr(regs->ax & 0xff);
- if (vsyscall_nr < 0) {
+ /*
+ * x86-ism here: regs->ip points to the instruction after the int 0xcc,
+ * and int 0xcc is two bytes long.
+ */
+ if (!is_vsyscall_entry(regs->ip - 2)) {
warn_bad_vsyscall(KERN_WARNING, regs, "illegal int 0xcc "
"(exploit attempt?)");
goto sigsegv;
}
+ vsyscall_nr = vsyscall_entry_nr(regs->ip - 2);

- if (regs->ip - 2 != vsyscall_intcc_addr(vsyscall_nr)) {
- if (in_vsyscall_page(regs->ip - 2)) {
- /* This should not be possible. */
- warn_bad_vsyscall(KERN_WARNING, regs,
- "int 0xcc bogus magic "
- "(exploit attempt?)");
- goto sigsegv;
- } else {
- /*
- * We allow the call because tools like ThreadSpotter
- * might copy the int 0xcc instruction to user memory.
- * We make it annoying, though, to try to persuade
- * the authors to stop doing that...
- */
- warn_bad_vsyscall(KERN_WARNING, regs,
- "int 0xcc in user code "
- "(exploit attempt? legacy "
- "instrumented code?)");
- }
+ if (get_user(caller, (unsigned long __user *)regs->sp) != 0) {
+ warn_bad_vsyscall(KERN_WARNING, regs, "int 0xcc with bad stack "
+ "(exploit attempt?)");
+ goto sigsegv;
}

tsk = current;
- if (seccomp_mode(&tsk->seccomp)) {
+ if (seccomp_mode(&tsk->seccomp))
do_exit(SIGKILL);
- goto out;
- }

switch (vsyscall_nr) {
case 0:
@@ -202,12 +169,8 @@ void dotraplinkage do_emulate_vsyscall(struct pt_regs *regs, long error_code)
(struct timezone __user *)regs->si);
break;

+#ifndef CONFIG_COMPAT_VSYSCALLS
case 1:
-#ifdef CONFIG_COMPAT_VSYSCALLS
- warn_bad_vsyscall(KERN_WARNING, regs, "bogus time() vsyscall "
- "emulation (exploit attempt?)");
- goto sigsegv;
-#else
vsyscall_name = "time";
ret = sys_time((time_t __user *)regs->di);
break;
@@ -221,6 +184,11 @@ void dotraplinkage do_emulate_vsyscall(struct pt_regs *regs, long error_code)
break;

default:
+ /*
+ * If we get here, then vsyscall_nr indicates that int 0xcc
+ * happened at an address in the vsyscall page that doesn't
+ * contain int 0xcc. That can't happen.
+ */
BUG();
}

@@ -240,9 +208,6 @@ void dotraplinkage do_emulate_vsyscall(struct pt_regs *regs, long error_code)
regs->ax = ret;

if (__ratelimit(&rs)) {
- unsigned long caller;
- if (get_user(caller, (unsigned long __user *)regs->sp))
- caller = 0; /* no need to crash on this fault. */
printk(KERN_INFO "%s[%d] emulated legacy vsyscall %s(); "
"upgrade your code to avoid a performance hit. "
"ip:%lx sp:%lx caller:%lx",
@@ -253,7 +218,10 @@ void dotraplinkage do_emulate_vsyscall(struct pt_regs *regs, long error_code)
printk("\n");
}

-out:
+ /* Emulate a ret instruction. */
+ regs->ip = caller;
+ regs->sp += 8;
+
local_irq_disable();
return;

diff --git a/arch/x86/kernel/vsyscall_emu_64.S b/arch/x86/kernel/vsyscall_emu_64.S
index 2d53e26..5658d42 100644
--- a/arch/x86/kernel/vsyscall_emu_64.S
+++ b/arch/x86/kernel/vsyscall_emu_64.S
@@ -7,36 +7,21 @@
#include <linux/linkage.h>
#include <asm/irq_vectors.h>

-/*
- * These magic incantations are chosen so that they fault if entered anywhere
- * other than an instruction boundary. The movb instruction is two bytes, and
- * the int imm8 instruction is also two bytes, so the only misaligned places
- * to enter are the immediate values for the two instructions. 0xcc is int3
- * (always faults), 0xce is into (faults on x64-64, and 32-bit code can't get
- * here), and 0xf0 is lock (lock int is invalid).
- *
- * The unused parts of the page are filled with 0xcc by the linker script.
- */
+/* The unused parts of the page are filled with 0xcc by the linker script. */

.section .vsyscall_0, "a"
ENTRY(vsyscall_0)
- movb $0xcc, %al
int $VSYSCALL_EMU_VECTOR
- ret
END(vsyscall_0)

#ifndef CONFIG_COMPAT_VSYSCALLS
.section .vsyscall_1, "a"
ENTRY(vsyscall_1)
- movb $0xce, %al
int $VSYSCALL_EMU_VECTOR
- ret
END(vsyscall_1)
#endif

.section .vsyscall_2, "a"
ENTRY(vsyscall_2)
- movb $0xf0, %al
int $VSYSCALL_EMU_VECTOR
- ret
END(vsyscall_2)

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