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SubjectRe: [PATCH v2 1/8] x86-64: Improve vsyscall emulation CS and RIP handling
On Mon, Jul 11, 2011 at 06:20:50PM -0400, Andrew Lutomirski wrote:
> > I'm wondering: why don't you make this function return negative value on
> > error, i.e. -EINVAL and the vsyscall number on success so that you can
> > get rid of returning it through the arg pointer?
> >
> > Then at the callsite you can do:
> >
> >        vsyscall_nr = addr_to_vsyscall_nr(addr);
> >        if (vsyscall_nr < 0)
> >                warn_bad_vsyscall(...)
>
> Because I don't want a warning about ret being used without being initialized.

not if you preinit it...

> With the code in this patch, the compiler is smart enough to figure
> out that either vsyscall_nr is 0, 1, or 2 or that the EINVAL branch is
> taken. I'll see if it works the other way.

here's what i mean, I changed your patch a bit:
--

From 2bb6c706cd6cf49ce5baec670bd01b53d2ea6f19 Mon Sep 17 00:00:00 2001
From: Andy Lutomirski <luto@MIT.EDU>
Date: Sat, 9 Jul 2011 23:22:08 -0400
Subject: [PATCH] x86-64: Improve vsyscall emulation CS and RIP handling

Two fixes here:
- Send SIGSEGV if called from compat code or with a funny CS.
- Don't BUG on impossible addresses.

This patch also removes an unused variable.

Signed-off-by: Andy Lutomirski <luto@mit.edu>
---
arch/x86/include/asm/vsyscall.h | 12 -------
arch/x86/kernel/vsyscall_64.c | 62 +++++++++++++++++++++++++-------------
2 files changed, 41 insertions(+), 33 deletions(-)

diff --git a/arch/x86/include/asm/vsyscall.h b/arch/x86/include/asm/vsyscall.h
index bb710cb..d555973 100644
--- a/arch/x86/include/asm/vsyscall.h
+++ b/arch/x86/include/asm/vsyscall.h
@@ -31,18 +31,6 @@ extern struct timezone sys_tz;

extern void map_vsyscall(void);

-/* Emulation */
-
-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 & 0xC00UL) >> 10;
-}
-
#endif /* __KERNEL__ */

#endif /* _ASM_X86_VSYSCALL_H */
diff --git a/arch/x86/kernel/vsyscall_64.c b/arch/x86/kernel/vsyscall_64.c
index 10cd8ac..2110cf2 100644
--- a/arch/x86/kernel/vsyscall_64.c
+++ b/arch/x86/kernel/vsyscall_64.c
@@ -38,6 +38,7 @@

#include <asm/vsyscall.h>
#include <asm/pgtable.h>
+#include <asm/compat.h>
#include <asm/page.h>
#include <asm/unistd.h>
#include <asm/fixmap.h>
@@ -97,33 +98,63 @@ 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\n",
+ printk("%s%s[%d] %s ip:%lx cs:%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);
+ message, regs->ip - 2, regs->cs,
+ regs->sp, regs->ax, regs->si, regs->di);
+}
+
+static int addr_to_vsyscall_nr(unsigned long addr)
+{
+ int vsyscall = -EINVAL;
+
+ if ((addr & ~0xC00UL) != VSYSCALL_START)
+ return -EINVAL;
+
+ vsyscall = (addr & 0xC00UL) >> 10;
+ if (vsyscall < 0 || vsyscall >= 3)
+ return -EINVAL;
+
+ return vsyscall;
}

void dotraplinkage do_emulate_vsyscall(struct pt_regs *regs, long error_code)
{
- const char *vsyscall_name;
struct task_struct *tsk;
unsigned long caller;
int vsyscall_nr;
- long ret;
-
- /* Kernel code must never get here. */
- BUG_ON(!user_mode(regs));
+ long ret = -EFAULT;

local_irq_enable();

/*
+ * Real 64-bit user mode code has cs == __USER_CS. Anything else
+ * is bogus.
+ */
+ if (regs->cs != __USER_CS) {
+ /*
+ * If we trapped from kernel mode, we might as well OOPS now
+ * instead of returning to some random address and OOPSing
+ * then.
+ */
+ BUG_ON(!user_mode(regs));
+
+ /* Compat mode and non-compat 32-bit CS should both segfault. */
+ warn_bad_vsyscall(KERN_WARNING, regs,
+ "illegal int 0xcc from 32-bit mode");
+ goto sigsegv;
+ }
+
+ /*
* 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?)");
+ vsyscall_nr = addr_to_vsyscall_nr(regs->ip - 2);
+ if (vsyscall_nr < 0) {
+ warn_bad_vsyscall(KERN_WARNING, regs,
+ "illegal int 0xcc (exploit attempt?)");
goto sigsegv;
}
- vsyscall_nr = vsyscall_entry_nr(regs->ip - 2);

if (get_user(caller, (unsigned long __user *)regs->sp) != 0) {
warn_bad_vsyscall(KERN_WARNING, regs, "int 0xcc with bad stack (exploit attempt?)");
@@ -136,31 +167,20 @@ void dotraplinkage do_emulate_vsyscall(struct pt_regs *regs, long error_code)

switch (vsyscall_nr) {
case 0:
- vsyscall_name = "gettimeofday";
ret = sys_gettimeofday(
(struct timeval __user *)regs->di,
(struct timezone __user *)regs->si);
break;

case 1:
- vsyscall_name = "time";
ret = sys_time((time_t __user *)regs->di);
break;

case 2:
- vsyscall_name = "getcpu";
ret = sys_getcpu((unsigned __user *)regs->di,
(unsigned __user *)regs->si,
0);
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();
}

if (ret == -EFAULT) {
--
1.7.5.3.401.gfb674



--
Regards/Gruss,
Boris.
--
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