+memory model folks because this seems like a pretty sharp corner On Wed, Apr 1, 2020 at 10:50 PM Eric W. Biederman <ebiederm@xxxxxxxxxxxx> wrote: > Replace the 32bit exec_id with a 64bit exec_id to make it impossible > to wrap the exec_id counter. With care an attacker can cause exec_id > wrap and send arbitrary signals to a newly exec'd parent. This > bypasses the signal sending checks if the parent changes their > credentials during exec. > > The severity of this problem can been seen that in my limited testing > of a 32bit exec_id it can take as little as 19s to exec 65536 times. > Which means that it can take as little as 14 days to wrap a 32bit > exec_id. Adam Zabrocki has succeeded wrapping the self_exe_id in 7 > days. Even my slower timing is in the uptime of a typical server. > Which means self_exec_id is simply a speed bump today, and if exec > gets noticably faster self_exec_id won't even be a speed bump. > > Extending self_exec_id to 64bits introduces a problem on 32bit > architectures where reading self_exec_id is no longer atomic and can > take two read instructions. Which means that is is possible to hit > a window where the read value of exec_id does not match the written > value. So with very lucky timing after this change this still > remains expoiltable. > > I have updated the update of exec_id on exec to use WRITE_ONCE > and the read of exec_id in do_notify_parent to use READ_ONCE > to make it clear that there is no locking between these two > locations. > > Link: https://lore.kernel.org/kernel-hardening/20200324215049.GA3710@xxxxxxxxxx > Fixes: 2.3.23pre2 > Cc: stable@xxxxxxxxxxxxxxx > Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx> > --- > > Linus would you prefer to take this patch directly or I could put it in > a brach and send you a pull request. > > fs/exec.c | 2 +- > include/linux/sched.h | 4 ++-- > kernel/signal.c | 2 +- > 3 files changed, 4 insertions(+), 4 deletions(-) > > diff --git a/fs/exec.c b/fs/exec.c > index 0e46ec57fe0a..d55710a36056 100644 > --- a/fs/exec.c > +++ b/fs/exec.c > @@ -1413,7 +1413,7 @@ void setup_new_exec(struct linux_binprm * bprm) > > /* An exec changes our domain. We are no longer part of the thread > group */ > - current->self_exec_id++; > + WRITE_ONCE(current->self_exec_id, current->self_exec_id + 1); GCC will generate code for this without complaining, but I think it'll probably generate a tearing store on 32-bit platforms: $ cat volatile-8.c typedef unsigned long long u64; static volatile u64 n; void blah(void) { n = n + 1; } $ gcc -O2 -m32 -c -o volatile-8.o volatile-8.c -Wall $ objdump --disassemble=blah volatile-8.o [...] b: 8b 81 00 00 00 00 mov 0x0(%ecx),%eax 11: 8b 91 04 00 00 00 mov 0x4(%ecx),%edx 17: 83 c0 01 add $0x1,%eax 1a: 83 d2 00 adc $0x0,%edx 1d: 89 81 00 00 00 00 mov %eax,0x0(%ecx) 23: 89 91 04 00 00 00 mov %edx,0x4(%ecx) [...] $ You could maybe use atomic64_t to work around that? atomic64_read() and atomic64_set() should be straightforward READ_ONCE() / WRITE_ONCE() on 64-bit systems while compiling into something more complicated on 32-bit. > flush_signal_handlers(current, 0); > } > EXPORT_SYMBOL(setup_new_exec); > diff --git a/include/linux/sched.h b/include/linux/sched.h > index 04278493bf15..0323e4f0982a 100644 > --- a/include/linux/sched.h > +++ b/include/linux/sched.h > @@ -939,8 +939,8 @@ struct task_struct { > struct seccomp seccomp; > > /* Thread group tracking: */ > - u32 parent_exec_id; > - u32 self_exec_id; > + u64 parent_exec_id; > + u64 self_exec_id; > > /* Protection against (de-)allocation: mm, files, fs, tty, keyrings, mems_allowed, mempolicy: */ > spinlock_t alloc_lock; > diff --git a/kernel/signal.c b/kernel/signal.c > index 9ad8dea93dbb..5383b562df85 100644 > --- a/kernel/signal.c > +++ b/kernel/signal.c > @@ -1926,7 +1926,7 @@ bool do_notify_parent(struct task_struct *tsk, int sig) > * This is only possible if parent == real_parent. > * Check if it has changed security domain. > */ > - if (tsk->parent_exec_id != tsk->parent->self_exec_id) > + if (tsk->parent_exec_id != READ_ONCE(tsk->parent->self_exec_id)) > sig = SIGCHLD; > } > > -- > 2.20.1 >
