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commit 285a76bb2c upstream.
The <asm/uaccess.h> header has a problem with put_user(a, ptr) if
the 'a' is not a simple variable, such as a function. This can lead
to the compiler producing code as so:
1: enable_user_access()
2: evaluate 'a' into register 'r'
3: put 'r' to 'ptr'
4: disable_user_acess()
The issue is that 'a' is now being evaluated with the user memory
protections disabled. So we try and force the evaulation by assigning
'x' to __val at the start, and hoping the compiler barriers in
enable_user_access() do the job of ordering step 2 before step 1.
This has shown up in a bug where 'a' sleeps and thus schedules out
and loses the SR_SUM flag. This isn't sufficient to fully fix, but
should reduce the window of opportunity. The first instance of this
we found is in scheudle_tail() where the code does:
$ less -N kernel/sched/core.c
4263 if (current->set_child_tid)
4264 put_user(task_pid_vnr(current), current->set_child_tid);
Here, the task_pid_vnr(current) is called within the block that has
enabled the user memory access. This can be made worse with KASAN
which makes task_pid_vnr() a rather large call with plenty of
opportunity to sleep.
Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Reported-by: syzbot+e74b94fe601ab9552d69@syzkaller.appspotmail.com
Suggested-by: Arnd Bergman <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
--
Changes since v1:
- fixed formatting and updated the patch description with more info
Changes since v2:
- fixed commenting on __put_user() (schwab@linux-m68k.org)
Change since v3:
- fixed RFC in patch title. Should be ready to merge.
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>
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.