projects/linux·commit 3fb2f423

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

3fb2f423 · Andy Lutomirski <[email protected]> · 2014-12-17 14:48:30 -08:00

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 <[email protected]>
Acked-by: Thomas Gleixner <[email protected]>
Cc: [email protected] # Only if 0e58af4e1d21 is backported
Cc: Linus Torvalds <[email protected]>
Link: http://lkml.kernel.org/r/d7875b60e28c512f6a6fc0baf5714d58e7eaadbb.1418856405.git.luto@amacapital.net
Signed-off-by: Ingo Molnar <[email protected]>
Commit
3fb2f4237bb452eb4e98f6a5dbd5a445b4fed9d0
Parents
0e58af4e

Changes

2 files changed · +7 −6

FileStatusAddDelContent
arch/x86/include/uapi/asm/ldt.hmodified+7−0View file
arch/x86/kernel/tls.cmodified+0−6View file

arch/x86/include/uapi/asm/ldt.h

View file
diff --git a/arch/x86/include/uapi/asm/ldt.h b/arch/x86/include/uapi/asm/ldt.h
index 46727eb..6e1aaf7 100644
--- a/arch/x86/include/uapi/asm/ldt.h
+++ b/arch/x86/include/uapi/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
};

arch/x86/kernel/tls.c

View file
diff --git a/arch/x86/kernel/tls.c b/arch/x86/kernel/tls.c
index 3e551ee..4e942f3 100644
--- a/arch/x86/kernel/tls.c
+++ b/arch/x86/kernel/tls.c
@@ -55,12 +55,6 @@ 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;
}