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SubjectRe: [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@vger.kernel.org
> Fixes: 457b9a6f09f0 ("Staging: android: add binder driver")
> Signed-off-by: Jann Horn <jannh@google.com>
> Acked-by: Todd Kjos <tkjos@google.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

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