From: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx> commit 8619e5bdeee8b2c685d686281f2d2a6017c4bc15 upstream. syzbot found that a thread can stall for minutes inside read_mem() or write_mem() after that thread was killed by SIGKILL [1]. Reading from iomem areas of /dev/mem can be slow, depending on the hardware. While reading 2GB at one read() is legal, delaying termination of killed thread for minutes is bad. Thus, allow reading/writing /dev/mem and /dev/kmem to be preemptible and killable. [ 1335.912419][T20577] read_mem: sz=4096 count=2134565632 [ 1335.943194][T20577] read_mem: sz=4096 count=2134561536 [ 1335.978280][T20577] read_mem: sz=4096 count=2134557440 [ 1336.011147][T20577] read_mem: sz=4096 count=2134553344 [ 1336.041897][T20577] read_mem: sz=4096 count=2134549248 Theoretically, reading/writing /dev/mem and /dev/kmem can become "interruptible". But this patch chose "killable". Future patch will make them "interruptible" so that we can revert to "killable" if some program regressed. [1] https://syzkaller.appspot.com/bug?id=a0e3436829698d5824231251fad9d8e998f94f5e Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx> Cc: stable <stable@xxxxxxxxxxxxxxx> Reported-by: syzbot <syzbot+8ab2d0f39fb79fe6ca40@xxxxxxxxxxxxxxxxxxxxxxxxx> Link: https://lore.kernel.org/r/1566825205-10703-1-git-send-email-penguin-kernel@xxxxxxxxxxxxxxxxxxx Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- drivers/char/mem.c | 21 +++++++++++++++++++++ 1 file changed, 21 insertions(+) --- a/drivers/char/mem.c +++ b/drivers/char/mem.c @@ -97,6 +97,13 @@ void __weak unxlate_dev_mem_ptr(phys_add } #endif +static inline bool should_stop_iteration(void) +{ + if (need_resched()) + cond_resched(); + return fatal_signal_pending(current); +} + /* * This funcion reads the *physical* memory. The f_pos points directly to the * memory location. @@ -175,6 +182,8 @@ static ssize_t read_mem(struct file *fil p += sz; count -= sz; read += sz; + if (should_stop_iteration()) + break; } kfree(bounce); @@ -251,6 +260,8 @@ static ssize_t write_mem(struct file *fi p += sz; count -= sz; written += sz; + if (should_stop_iteration()) + break; } *ppos += written; @@ -468,6 +479,10 @@ static ssize_t read_kmem(struct file *fi read += sz; low_count -= sz; count -= sz; + if (should_stop_iteration()) { + count = 0; + break; + } } } @@ -492,6 +507,8 @@ static ssize_t read_kmem(struct file *fi buf += sz; read += sz; p += sz; + if (should_stop_iteration()) + break; } free_page((unsigned long)kbuf); } @@ -544,6 +561,8 @@ static ssize_t do_write_kmem(unsigned lo p += sz; count -= sz; written += sz; + if (should_stop_iteration()) + break; } *ppos += written; @@ -595,6 +614,8 @@ static ssize_t write_kmem(struct file *f buf += sz; virtr += sz; p += sz; + if (should_stop_iteration()) + break; } free_page((unsigned long)kbuf); }
