On Mon, Sep 28, 2020 at 07:41:13PM +0530, Pintu Agarwal wrote: > On Mon, 28 Sep 2020 at 19:15, Pintu Agarwal <pintu.ping@xxxxxxxxx> wrote: > > > > On Sat, 26 Sep 2020 at 22:10, Kees Cook <keescook@xxxxxxxxxxxx> wrote: > > > > > > >> I wonder if this is an Android Common kernel? > > > > It uses the below kernel for 4.14: > > > > https://gitlab.com/quicla/kernel/msm-4.14/-/tree/LE.UM.3.4.2.r1.5 (or > > > > similar branch). > > > > > > Okay, so yes. And this appears to have the hashing of %p backported. I > > > cannot, however, explain why it's showing hashed pointers instead of > > > just NULL, though. > > > > > > It might be related to these commits but they're not in that kernel: > > > 3e5903eb9cff ("vsprintf: Prevent crash when dereferencing invalid pointers") > > > 7bd57fbc4a4d ("vsprintf: don't obfuscate NULL and error pointers") > > > > > > > ==> The case where symbol addresses are changing. > > > > > > > > kptr_restrict is set to 2 by default: > > > > / # cat /proc/sys/kernel/kptr_restrict > > > > 2 > > > > > > > > Basically, the goal is: > > > > * To understand how addresses are changing in 4.14 Kernel (without > > > > KASLR support)? > > > > * Is it possible to support the same in 4.9 Kernel ? > > > > > > Try setting kptr_restrict to 0 and see if the symbol addresses change? I > > > suspect Ard is correct: there's no KASLR here, just hashed pointers > > > behaving weird on an old non-stock kernel. :) > > > > > > > Okay. Thank you so much for your comments and suggestions. > > You mean to say, setting kptr_restrict to 0 may avoid changing symbol > > addresses in 4.14 ? > > And, sorry, I could not understand the thing about this "hashed pointers". > > How can I check this behavior in source code to understand better? > > Is it possible to give some reference ? > > I wanted to disable this hash pointer on 4.14 kernel and check the behavior. > > Also if possible, we would like to make this similar change on 4.9 > > kernel as well. > > > > Okay, I found these changes in 4.14 kernel: > https://gitlab.com/quicla/kernel/msm-4.14/-/commit/e63732dbfe017aa0dbabac9d096b5fde8afbd395 That's an out-of-tree patch. The one that was backported to 4.14 from 4.15 that does %p hashing is this one: https://gitlab.com/quicla/kernel/msm-4.14/-/commit/ad67b74d2469d9b82aaa572d76474c95bc484d57 > Are we talking about this? > I cound not find this in 4.9 kernel. Note that I still can't explain what you're seeing. /proc/kallsyms shouldn't be affected by these patches -- I would expect it to either report the true value or 00s. > I will disable kptr_restrict and check or, I will enable > CONFIG_DEBUG_CONSOLE_UNHASHED_POINTERS and check. I would start with kptr_restrict=0 and see if that changes the contents of /proc/kallsyms. If it does, then it is the hashing. If it doesn't, well, we're back to square one. :) -- Kees Cook _______________________________________________ Kernelnewbies mailing list Kernelnewbies@xxxxxxxxxxxxxxxxx https://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
