Commit 3fb2f423 authored Dec 17, 2014 by Andy Lutomirski Committed by Ingo Molnar Dec 18, 2014

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



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: 0e58af4e ("x86/tls: Disallow unusual TLS segments")
Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org # Only if 0e58af4e is backported
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>

parent 0e58af4e

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

+7 −0

Original line number	Diff line number	Diff line
		@@ -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

+0 −6

Original line number	Diff line number	Diff line
		@@ -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;
		}

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