linux-kernel - Re: [PATCH] aio: Fix io_pgetevents() struct __compat_aio

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Date:   Thu, 22 Aug 2019 13:53:30 -0400
From:   Jeff Moyer <jmoyer@...hat.com>
To:     Guillem Jover <guillem@...rons.org>
Cc:     linux-aio@...ck.org, Christoph Hellwig <hch@....de>,
        Benjamin LaHaise <bcrl@...ck.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] aio: Fix io_pgetevents() struct __compat_aio_sigset layout

Guillem Jover <guillem@...rons.org> writes:

> This type is used to pass the sigset_t from userland to the kernel,
> but it was using the kernel native pointer type for the member
> representing the compat userland pointer to the userland sigset_t.
>
> This messes up the layout, and makes the kernel eat up both the
> userland pointer and the size members into the kernel pointer, and
> then reads garbage into the kernel sigsetsize. Which makes the sigset_t
> size consistency check fail, and consequently the syscall always
> returns -EINVAL.
>
> This breaks both libaio and strace on 32-bit userland running on 64-bit
> kernels. And there are apparently no users in the wild of the current
> broken layout (at least according to codesearch.debian.org and a brief
> check over github.com search). So it looks safe to fix this directly
> in the kernel, instead of either letting userland deal with this
> permanently with the additional overhead or trying to make the syscall
> infer what layout userland used, even though this is also being worked
> around in libaio to temporarily cope with kernels that have not yet
> been fixed.
>
> We use a proper compat_uptr_t instead of a compat_sigset_t pointer.
>
> Fixes: 7a074e96 ("aio: implement io_pgetevents")
> Signed-off-by: Guillem Jover <guillem@...rons.org>

Looks good, thanks for finding and fixing this!

Reviewed-by: Jeff Moyer <jmoyer@...hat.com>

> ---
>  fs/aio.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/fs/aio.c b/fs/aio.c
> index 01e0fb9ae45a..056f291bc66f 100644
> --- a/fs/aio.c
> +++ b/fs/aio.c
> @@ -2179,7 +2179,7 @@ SYSCALL_DEFINE5(io_getevents_time32, __u32, ctx_id,
>  #ifdef CONFIG_COMPAT
>  
>  struct __compat_aio_sigset {
> -	compat_sigset_t __user	*sigmask;
> +	compat_uptr_t		sigmask;
>  	compat_size_t		sigsetsize;
>  };
>  
> @@ -2204,7 +2204,7 @@ COMPAT_SYSCALL_DEFINE6(io_pgetevents,
>  	if (usig && copy_from_user(&ksig, usig, sizeof(ksig)))
>  		return -EFAULT;
>  
> -	ret = set_compat_user_sigmask(ksig.sigmask, ksig.sigsetsize);
> +	ret = set_compat_user_sigmask(compat_ptr(ksig.sigmask), ksig.sigsetsize);
>  	if (ret)
>  		return ret;
>  
> @@ -2239,7 +2239,7 @@ COMPAT_SYSCALL_DEFINE6(io_pgetevents_time64,
>  	if (usig && copy_from_user(&ksig, usig, sizeof(ksig)))
>  		return -EFAULT;
>  
> -	ret = set_compat_user_sigmask(ksig.sigmask, ksig.sigsetsize);
> +	ret = set_compat_user_sigmask(compat_ptr(ksig.sigmask), ksig.sigsetsize);
>  	if (ret)
>  		return ret;