Messages in this thread | | | From | Linus Torvalds <> | Date | Mon, 4 Mar 2019 10:59:22 -0800 | Subject | Re: [uaccess] 780464aed0: WARNING:at_arch/x86/include/asm/uaccess.h:#strnlen_user/0x |
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On Mon, Mar 4, 2019 at 1:06 AM Masami Hiramatsu <mhiramat@kernel.org> wrote: > > On Sun, 3 Mar 2019 18:37:59 -0800 > Linus Torvalds <torvalds@linux-foundation.org> wrote: > > We've had this before. We've gotten rid of the actual "use system > > calls", but we still have some of the init sequence in particular just > > calling the wrappers instead. > > Are those safe if we are in init sequence?
Yes, they are, it runs with set_fs(KERNEL_DS).
But the patches made that now complain about copying from non-user space, even though it's fine.
Basically, "strncpy_from_user()" shouldn't use "user_access_ok()", since it actually can take a kernel address (with set_fs()).
Your "unsafe" version for tracing that actually sets "set_fs(USER_DS)" is thje only thing that should use that helper.
> > And yes, ksys_mount() takes __user pointers. > > > > It would be a lot better to use "do_mount()", which is the interface > > that takes actual "char *" pointers. > > Unfortunately, it still takes a __user pointer.
Ahh, yes, the name remains in user space.
Besides, I'm sure you'd just hit other cases instead where people use set_fs() and copy strings.
> So what we need is > > long do_mount(const char *dev_name, struct path *dir_path, > const char *type_page, unsigned long flags, void *data_page) > > or introduce kern_do_mount()?
It's actually fairly painful. Particularly because of that "void *data_page".
Your second email with "Would this work?" helper function _wopuldn't_ work correctly, exactly because you passed in a regular string to the data page.
Also, I don't want to see code that replaces the unconditional "copy path from user space" with a conditional "do we have path in kernel space".
So together with the whole "uyou'll hit other peoblems anyway", I don't think this is a good approach.
I think you simply need to have a separate "unsafe_strncpy()" function, and not change the existing "strncpy_from_user()".
Linus
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