Commit 46561c39 authored Nov 11, 2015 by Dave Hansen Committed by Ingo Molnar Nov 12, 2015

x86/mpx: Do proper get_user() when running 32-bit binaries on 64-bit kernels



When you call get_user(foo, bar), you effectively do a

	copy_from_user(&foo, bar, sizeof(*bar));

Note that the sizeof() is implicit.

When we reach out to userspace to try to zap an entire "bounds
table" we need to go read a "bounds directory entry" in order to
locate the table's address.  The size of a "directory entry"
depends on the binary being run and is always the size of a
pointer.

But, when we have a 64-bit kernel and a 32-bit application, the
directory entry is still only 32-bits long, but we fetch it with
a 64-bit pointer which makes get_user() does a 64-bit fetch.
Reading 4 extra bytes isn't harmful, unless we are at the end of
and run off the table.  It might also cause the zero page to get
faulted in unnecessarily even if you are not at the end.

Fix it up by doing a special 32-bit get_user() via a cast when
we have 32-bit userspace.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: <stable@vger.kernel.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Dave Hansen <dave@sr71.net>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20151111181931.3ACF6822@viggo.jf.intel.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>

parent 04633df0

arch/x86/mm/mpx.c

+24 −1

Original line number	Diff line number	Diff line
		@@ -585,6 +585,29 @@ static unsigned long mpx_bd_entry_to_bt_addr(struct mm_struct *mm,
		return bt_addr;
		}

		/*
		* We only want to do a 4-byte get_user() on 32-bit. Otherwise,
		* we might run off the end of the bounds table if we are on
		* a 64-bit kernel and try to get 8 bytes.
		*/
		int get_user_bd_entry(struct mm_struct mm, unsigned long bd_entry_ret,
		long __user *bd_entry_ptr)
		{
		u32 bd_entry_32;
		int ret;

		if (is_64bit_mm(mm))
		return get_user(*bd_entry_ret, bd_entry_ptr);

		/*
		* Note that get_user() uses the type of the pointer to
		* establish the size of the get, not the destination.
		*/
		ret = get_user(bd_entry_32, (u32 __user *)bd_entry_ptr);
		*bd_entry_ret = bd_entry_32;
		return ret;
		}

		/*
		* Get the base of bounds tables pointed by specific bounds
		* directory entry.
		@@ -605,7 +628,7 @@ static int get_bt_addr(struct mm_struct *mm,
		int need_write = 0;

		pagefault_disable();
		ret = get_user(bd_entry, bd_entry_ptr);
		ret = get_user_bd_entry(mm, &bd_entry, bd_entry_ptr);
		pagefault_enable();
		if (!ret)
		break;

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