linux-kernel - Re: [PATCH resend] binder: Prevent context manager from incrementing ref 0

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Date:   Fri, 10 Jul 2020 08:54:18 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Jann Horn <jannh@...gle.com>
Cc:     Arve Hjønnevåg <arve@...roid.com>,
        Todd Kjos <tkjos@...roid.com>,
        Martijn Coenen <maco@...roid.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Christian Brauner <christian@...uner.io>,
        devel@...verdev.osuosl.org, Mattias Nissler <mnissler@...gle.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH resend] binder: Prevent context manager from incrementing
 ref 0

On Fri, Jul 10, 2020 at 12:39:48AM +0200, Jann Horn wrote:
> Binder is designed such that a binder_proc never has references to
> itself. If this rule is violated, memory corruption can occur when a
> process sends a transaction to itself; see e.g.
> <https://syzkaller.appspot.com/bug?extid=09e05aba06723a94d43d>.
> 
> There is a remaining edgecase through which such a transaction-to-self
> can still occur from the context of a task with BINDER_SET_CONTEXT_MGR
> access:
> 
>  - task A opens /dev/binder twice, creating binder_proc instances P1
>    and P2
>  - P1 becomes context manager
>  - P2 calls ACQUIRE on the magic handle 0, allocating index 0 in its
>    handle table
>  - P1 dies (by closing the /dev/binder fd and waiting a bit)
>  - P2 becomes context manager
>  - P2 calls ACQUIRE on the magic handle 0, allocating index 1 in its
>    handle table
>    [this triggers a warning: "binder: 1974:1974 tried to acquire
>    reference to desc 0, got 1 instead"]
>  - task B opens /dev/binder once, creating binder_proc instance P3
>  - P3 calls P2 (via magic handle 0) with (void*)1 as argument (two-way
>    transaction)
>  - P2 receives the handle and uses it to call P3 (two-way transaction)
>  - P3 calls P2 (via magic handle 0) (two-way transaction)
>  - P2 calls P2 (via handle 1) (two-way transaction)
> 
> And then, if P2 does *NOT* accept the incoming transaction work, but
> instead closes the binder fd, we get a crash.
> 
> Solve it by preventing the context manager from using ACQUIRE on ref 0.
> There shouldn't be any legitimate reason for the context manager to do
> that.
> 
> Additionally, print a warning if someone manages to find another way to
> trigger a transaction-to-self bug in the future.
> 
> Cc: stable@...r.kernel.org
> Fixes: 457b9a6f09f0 ("Staging: android: add binder driver")
> Signed-off-by: Jann Horn <jannh@...gle.com>
> Acked-by: Todd Kjos <tkjos@...gle.com>
> ---
> sending again because I forgot to CC LKML the first time... sorry about
> the spam.

This spits out a bunch of warnings when built, how did it work on your
end?

drivers/android/binder.c: In function ‘binder_thread_write’:
./include/linux/kern_levels.h:5:18: warning: format ‘%d’ expects a matching ‘int’ argument [-Wformat=]
    5 | #define KERN_SOH "\001"  /* ASCII Start Of Header */
      |                  ^~~~~~
./include/linux/printk.h:507:10: note: in definition of macro ‘printk_ratelimited’
  507 |   printk(fmt, ##__VA_ARGS__);    \
      |          ^~~
./include/linux/kern_levels.h:14:19: note: in expansion of macro ‘KERN_SOH’
   14 | #define KERN_INFO KERN_SOH "6" /* informational */
      |                   ^~~~~~~~
./include/linux/printk.h:527:21: note: in expansion of macro ‘KERN_INFO’
  527 |  printk_ratelimited(KERN_INFO pr_fmt(fmt), ##__VA_ARGS__)
      |                     ^~~~~~~~~
drivers/android/binder.c:147:4: note: in expansion of macro ‘pr_info_ratelimited’
  147 |    pr_info_ratelimited(x); \
      |    ^~~~~~~~~~~~~~~~~~~
drivers/android/binder.c:3646:7: note: in expansion of macro ‘binder_user_error’
 3646 |       binder_user_error("%d:%d context manager tried to acquire desc 0\n");
      |       ^~~~~~~~~~~~~~~~~
./include/linux/kern_levels.h:5:18: warning: format ‘%d’ expects a matching ‘int’ argument [-Wformat=]
    5 | #define KERN_SOH "\001"  /* ASCII Start Of Header */
      |                  ^~~~~~
./include/linux/printk.h:507:10: note: in definition of macro ‘printk_ratelimited’
  507 |   printk(fmt, ##__VA_ARGS__);    \
      |          ^~~
./include/linux/kern_levels.h:14:19: note: in expansion of macro ‘KERN_SOH’
   14 | #define KERN_INFO KERN_SOH "6" /* informational */
      |                   ^~~~~~~~
./include/linux/printk.h:527:21: note: in expansion of macro ‘KERN_INFO’
  527 |  printk_ratelimited(KERN_INFO pr_fmt(fmt), ##__VA_ARGS__)
      |                     ^~~~~~~~~
drivers/android/binder.c:147:4: note: in expansion of macro ‘pr_info_ratelimited’
  147 |    pr_info_ratelimited(x); \
      |    ^~~~~~~~~~~~~~~~~~~
drivers/android/binder.c:3646:7: note: in expansion of macro ‘binder_user_error’
 3646 |       binder_user_error("%d:%d context manager tried to acquire desc 0\n");
      |       ^~~~~~~~~~~~~~~~~


thanks,

greg k-h