x86/tls: Don't validate lm in set_thread_area() after all

commit 3fb2f4237bb452eb4e98f6a5dbd5a445b4fed9d0 upstream.

It turns out that there's a lurking ABI issue.  GCC, when
compiling this in a 32-bit program:

struct user_desc desc = {
	.entry_number    = idx,
	.base_addr       = base,
	.limit           = 0xfffff,
	.seg_32bit       = 1,
	.contents        = 0, /* Data, grow-up */
	.read_exec_only  = 0,
	.limit_in_pages  = 1,
	.seg_not_present = 0,
	.useable         = 0,
};

will leave .lm uninitialized.  This means that anything in the
kernel that reads user_desc.lm for 32-bit tasks is unreliable.

Revert the .lm check in set_thread_area().  The value never did
anything in the first place.

Fixes: 0e58af4e1d21 ("x86/tls: Disallow unusual TLS segments")
Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/d7875b60e28c512f6a6fc0baf5714d58e7eaadbb.1418856405.git.luto@amacapital.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
[bwh: Backported to 3.2: adjust filename]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/arch/x86/include/asm/ldt.h b/arch/x86/include/asm/ldt.h
index 46727eb..6e1aaf7 100644 (file)
--- a/arch/x86/include/asm/ldt.h
+++ b/arch/x86/include/asm/ldt.h
@@ -28,6 +28,13 @@ struct user_desc {
        unsigned int  seg_not_present:1;
        unsigned int  useable:1;
 #ifdef __x86_64__
+       /*
+        * Because this bit is not present in 32-bit user code, user
+        * programs can pass uninitialized values here.  Therefore, in
+        * any context in which a user_desc comes from a 32-bit program,
+        * the kernel must act as though lm == 0, regardless of the
+        * actual value.
+        */
        unsigned int  lm:1;
 #endif
 };

diff --git a/arch/x86/kernel/tls.c b/arch/x86/kernel/tls.c
index e7d6e9f..59e07af 100644 (file)
--- a/arch/x86/kernel/tls.c
+++ b/arch/x86/kernel/tls.c
@@ -56,12 +56,6 @@ static bool tls_desc_okay(const struct user_desc *info)
        if (info->seg_not_present)
                return false;
 
-#ifdef CONFIG_X86_64
-       /* The L bit makes no sense for data. */
-       if (info->lm)
-               return false;
-#endif
-
        return true;
 }