linux-kernel - Re: [PATCH 4/4] file: Fix file descriptor leak in copy_fd

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Date:   Sat, 26 Mar 2022 17:17:32 +0300
From:   Alexey Khoroshilov <khoroshilov@...ras.ru>
To:     Fedor Pchelkin <aissur0002@...il.com>,
        Alexander Viro <viro@...iv.linux.org.uk>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 4/4] file: Fix file descriptor leak in copy_fd_bitmaps()

Looks like bfp has a set of macro suitable for such cases:

#define BITS_PER_BYTE_MASKED(bits) ((bits) & BITS_PER_BYTE_MASK)
#define BITS_ROUNDDOWN_BYTES(bits) ((bits) >> 3)
#define BITS_ROUNDUP_BYTES(bits) \
	(BITS_ROUNDDOWN_BYTES(bits) + !!BITS_PER_BYTE_MASKED(bits))

May be it makes sense to move them to a generic header and to use here?

--
Alexey Khoroshilov


On 26.03.2022 14:40, Fedor Pchelkin wrote:
> If count argument in copy_fd_bitmaps() is not a multiple of
> BITS_PER_BYTE, then one byte is lost and is not used in further
> manipulations with cpy value in memcpy() and memset()
> causing a leak. The leak was introduced with close_range() call
> using CLOSE_RANGE_UNSHARE flag.
> 
> The patch suggests implementing an indicator (named add_byte)
> of count being multiple of BITS_PER_BYTE and adding it to the
> cpy value.
> 
> Found by Syzkaller (https://github.com/google/syzkaller).
> 
> Signed-off-by: Fedor Pchelkin <aissur0002@...il.com>
> Signed-off-by: Alexey Khoroshilov <khoroshilov@...ras.ru>
> ---
>  fs/file.c | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/fs/file.c b/fs/file.c
> index 3ef1479df203..3c64a6423604 100644
> --- a/fs/file.c
> +++ b/fs/file.c
> @@ -56,10 +56,8 @@ static void copy_fd_bitmaps(struct fdtable *nfdt, struct fdtable *ofdt,
>  {
>  	unsigned int cpy, set;
>  	unsigned int add_byte = 0;
> -	
>  	if (count % BITS_PER_BYTE != 0)
>  		add_byte = 1;
> -	
>  	cpy = count / BITS_PER_BYTE + add_byte;
>  	set = (nfdt->max_fds - count) / BITS_PER_BYTE;
>  	memcpy(nfdt->open_fds, ofdt->open_fds, cpy);
>