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| Filename | Latest commit message | Latest commit date |
|---|---|---|
75bbe7bd90
[ Upstream commit 75aab4e10ae6a4593a60f66d13de755d4e91f400 ]
The page recycle code, incorrectly, relied on that a page fragment
could not be freed inside xdp_do_redirect(). This assumption leads to
that page fragments that are used by the stack/XDP redirect can be
reused and overwritten.
To avoid this, store the page count prior invoking xdp_do_redirect().
Longer explanation:
Intel NICs have a recycle mechanism. The main idea is that a page is
split into two parts. One part is owned by the driver, one part might
be owned by someone else, such as the stack.
t0: Page is allocated, and put on the Rx ring
+---------------
used by NIC ->| upper buffer
(rx_buffer) +---------------
| lower buffer
+---------------
page count == USHRT_MAX
rx_buffer->pagecnt_bias == USHRT_MAX
t1: Buffer is received, and passed to the stack (e.g.)
+---------------
| upper buff (skb)
+---------------
used by NIC ->| lower buffer
(rx_buffer) +---------------
page count == USHRT_MAX
rx_buffer->pagecnt_bias == USHRT_MAX - 1
t2: Buffer is received, and redirected
+---------------
| upper buff (skb)
+---------------
used by NIC ->| lower buffer
(rx_buffer) +---------------
Now, prior calling xdp_do_redirect():
page count == USHRT_MAX
rx_buffer->pagecnt_bias == USHRT_MAX - 2
This means that buffer *cannot* be flipped/reused, because the skb is
still using it.
The problem arises when xdp_do_redirect() actually frees the
segment. Then we get:
page count == USHRT_MAX - 1
rx_buffer->pagecnt_bias == USHRT_MAX - 2
From a recycle perspective, the buffer can be flipped and reused,
which means that the skb data area is passed to the Rx HW ring!
To work around this, the page count is stored prior calling
xdp_do_redirect().
Note that this is not optimal, since the NIC could actually reuse the
"lower buffer" again. However, then we need to track whether
XDP_REDIRECT consumed the buffer or not.
Fixes:
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| README | ||
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.