Re: [PATCH 15/17] x86, mpx: do 32-bit-only cmpxchg for 32-bit apps

Borislav Petkov
2015-03-27T17:31:08Z

On Thu, Mar 26, 2015 at 11:33:53AM -0700, Dave Hansen wrote:
>
> From: Dave Hansen <dave.hansen@linux.intel.com>
>
> user_atomic_cmpxchg_inatomic() actually looks at sizeof(*ptr) to
> figure out how many bytes to copy. If we run it on a 64-bit
> kernel with a 64-bit pointer, it will copy a 64-bit bounds
> directory entry. That's fine, except when we have 32-bit
> programs with 32-bit bounds directory entries and we only *want*
> 32-bits.
>
> This patch breaks the cmpxchg operation out in to its own
> function and performs the 32-bit type swizzling in there.
>
> Note, the "64-bit" version of this code _would_ work on a
> 32-bit-only kernel. The issue this patch addresses is only for
> when the kernel's 'long' is mismatched from the size of the
> bounds directory entry of the process we are working on.
>
> The new helper modifies 'actual_old_val' or returns an error.
> But gcc doesn't know this, so it warns about 'actual_old_val'
> being unused. Shut it up with an uninitialized_var().
>
> Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
> ---
>
> b/arch/x86/mm/mpx.c | 41 ++++++++++++++++++++++++++++++++++++-----
> 1 file changed, 36 insertions(+), 5 deletions(-)
>
> diff -puN arch/x86/mm/mpx.c~mpx-variable-sized-userspace-pokes arch/x86/mm/mpx.c
> --- a/arch/x86/mm/mpx.c~mpx-variable-sized-userspace-pokes 2015-03-26 11:27:33.927520821 -0700
> +++ b/arch/x86/mm/mpx.c 2015-03-26 11:27:33.931521001 -0700
> @@ -421,6 +421,35 @@ int mpx_disable_management(struct task_s
> return 0;
> }
>
> +static int mpx_cmpxchg_bd_entry(struct mm_struct *mm,
> + unsigned long *actual_old_val_ptr, long __user *bd_entry_addr,
> + unsigned long expected_old_val, unsigned long new_bd_entry)
> +{
> + int ret;
> + /*
> + * user_atomic_cmpxchg_inatomic() actually uses sizeof()
> + * the pointer thatt we pass to it to figure out how much
> + * data to cmpxchg. We have to be careful here not to
> + * pass a pointer to a 64-bit data type when we only want
> + * a 32-bit copy.
> + */
> + if (is_64bit_mm(mm)) {
> + ret = user_atomic_cmpxchg_inatomic(actual_old_val_ptr,
> + bd_entry_addr, expected_old_val, new_bd_entry);
> + } else {
> + u32 uninitialized_var(actual_old_val_32);
> + u32 expected_old_val_32 = expected_old_val;
> + u32 new_bd_entry_32 = new_bd_entry;
> + u32 __user *bd_entry_32 = (u32 __user *)bd_entry_addr;
> + ret = user_atomic_cmpxchg_inatomic(&actual_old_val_32,
> + bd_entry_32, expected_old_val_32,
> + new_bd_entry_32);

Hmm, I would've added a user_atomic_cmpxchg_inatomic_size() macro which
calls __user_atomic_cmpxchg_inatomic().

--
Regards/Gruss,
Boris.

ECO tip #101: Trim your mails when you reply.
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