Bug 215720 - brk() regression on AArch64 on static-pie binary -- issue with ASLR and a guard page?

[Thorsten Leemhuis <regressions@leemhuis.info>]

Hi, this is your Linux kernel regression tracker.

I noticed a regression report in bugzilla.kernel.org that afaics nobody
acted upon since it was reported about a week ago, that's why I decided
to forward it to the lists and the author of the culprit. To quote from
https://bugzilla.kernel.org/show_bug.cgi?id=215720:

>  Victor Stinner 2022-03-22 02:24:57 UTC
> 
> Created attachment 300597 [details]
> empty.c reproducer
> 
> I found a brk() syscall regression of Linux kernel 5.17 on AArch64.
> 
> A git bisect found the change "fs/binfmt_elf: use PT_LOAD p_align values for static PIE": commit 9630f0d60fec5fbcaa4435a66f75df1dc9704b66, changed related to the bz#215275.
> 
> Program to reproduce the bug, empty.c (attached to the issue):
> ---
> _Thread_local int var1 = 0;
> int main() {
>     volatile int x = 1;
>     var1 = x;
>     return 0;
> }
> ---
> 
> Build the program as a static PIE program:
> 
>     gcc -std=c11 -static-pie -g empty.c -o empty -O2
> 
> The program fails randomly, it takes 100 to 6000 runs to reproduce the crash.
> 
> Short shell loop to reproduce the crash:
> ---
> $ i=0; while true; do ./empty; rc=$?; i=$(($i + 1)); echo "$i:
> $(date): $rc"; if [ $rc -ne 0 ]; then break; fi; done
> (...)
> 159: Tue Mar 22 01:54:22 CET 2022: 0
> 160: Tue Mar 22 01:54:22 CET 2022: 0
> Segmentation fault (core dumped)
> 161: Tue Mar 22 01:54:22 CET 2022: 139
> ---
> 
> Disabling ASLR (write 0 to /proc/sys/kernel/randomize_va_space) works
> around the bug.
> 
> Rather than using "empty.c" program, the "ldconfig -V > /dev/null" command can be used: standard static-pie program.
> 
> strace when the program works:
> ---
> brk(NULL)                               = 0xaaaac3961000
> brk(0xaaaac3961b78)                     = 0xaaaac3961b78
> ---
> 
> strace when the bug occurs:
> ---
> brk(NULL)                               = 0xaaaabf3c3000
> brk(0xaaaabf3c3b78)                     = 0xaaaabf3c3000
> ---
> 
> The following test of the brk() syscall fails when the bug occurs:
> ---
> 	/* Check against existing mmap mappings. */
> 	next = find_vma(mm, oldbrk);
> 	if (next && newbrk + PAGE_SIZE > vm_start_gap(next))
> 		goto out;
> ---
> 
> Note: When the bug occurs, the program crash with SIGSEGV: the glibc __libc_setup_tls() function calls sbrk(2936) to allocate TLS variables, but it doesn't handle the memory allocation failure.
> 
> Note: At the beginning, I discovered this kernel regression while checking for Python
> buildbot failures on our Fedora Rawhide AArch64 machine.
> 
> * Fedora downstream issue: https://bugzilla.redhat.com/show_bug.cgi?id=2066147
> * Python issue: https://bugs.python.org/issue47078
> 
> [reply] [−] Comment 1 Victor Stinner 2022-03-22 02:41:00 UTC
> 
> See also the binutils issue: "p_align in ELF program headers should not exceed section alignment"
> https://sourceware.org/bugzilla/show_bug.cgi?id=28689
> 
> See also this old (kernel 4.18) fixed x86-64 kernel bug: "kernel: brk can grow the heap into the area reserved for the stack"
> https://bugzilla.redhat.com/show_bug.cgi?id=1749633


Could somebody take a look into this? Or was this discussed somewhere
else already? Or even fixed?

Anyway, to get this tracked:

#regzbot introduced: 9630f0d60fec5fbcaa4435a66f75df1dc9704b66
#regzbot from: Victor Stinner <vstinner@redhat.com>
#regzbot title: brk() regression on AArch64 on static-pie binary
#regzbot link: https://bugzilla.kernel.org/show_bug.cgi?id=215720

Ciao, Thorsten (wearing his 'the Linux kernel's regression tracker' hat)

P.S.: As the Linux kernel's regression tracker I'm getting a lot of
reports on my table. I can only look briefly into most of them and lack
knowledge about most of the areas they concern. I thus unfortunately
will sometimes get things wrong or miss something important. I hope
that's not the case here; if you think it is, don't hesitate to tell me
in a public reply, it's in everyone's interest to set the public record
straight.

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