On Fri, Aug 07, 2020 at 08:45:00PM +0200, Jann Horn wrote: > commit 4b836a1426cb0f1ef2a6e211d7e553221594f8fc upstream. > > 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@xxxxxxxxxxxxxxx > Fixes: 457b9a6f09f0 ("Staging: android: add binder driver") > Acked-by: Todd Kjos <tkjos@xxxxxxxxxx> > Signed-off-by: Jann Horn <jannh@xxxxxxxxxx> > Reviewed-by: Martijn Coenen <maco@xxxxxxxxxxx> > Link: https://lore.kernel.org/r/20200727120424.1627555-1-jannh@xxxxxxxxxx > Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> > [manual backport: remove fine-grained locking and error reporting that > don't exist in <=4.9] > Signed-off-by: Jann Horn <jannh@xxxxxxxxxx> Thanks for the backport, now queued up. greg k-h
