Hi, Thorsten here, the Linux kernel's regression tracker. Top-posting for once, to make this easily accessible to everyone. Eric, what's the status wrt. to this regression? Things from here look stalled, but I might be missing something. Ciao, Thorsten (wearing his 'the Linux kernel's regression tracker' hat) -- Everything you wanna know about Linux kernel regression tracking: https://linux-regtracking.leemhuis.info/about/#tldr If I did something stupid, please tell me, as explained on that page. #regzbot poke On 22.01.24 23:12, Kees Cook wrote: > On Mon, Jan 22, 2024 at 03:01:06PM -0600, Eric W. Biederman wrote: >> Kees Cook <keescook@xxxxxxxxxxxx> writes: >> >>> On Mon, Jan 22, 2024 at 10:43:59AM -0600, Eric W. Biederman wrote: >>>> Jan Bujak <j@xxxxxxx> writes: >>>> >>>>> Hi. >>>>> >>>>> I recently updated my kernel and one of my programs started segfaulting. >>>>> >>>>> The issue seems to be related to how the kernel interprets PT_LOAD headers; >>>>> consider the following program headers (from 'readelf' of my reproduction): >>>>> >>>>> Program Headers: >>>>> Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align >>>>> LOAD 0x001000 0x10000 0x10000 0x000010 0x000010 R 0x1000 >>>>> LOAD 0x002000 0x11000 0x11000 0x000010 0x000010 RW 0x1000 >>>>> LOAD 0x002010 0x11010 0x11010 0x000000 0x000004 RW 0x1000 >>>>> LOAD 0x003000 0x12000 0x12000 0x0000d2 0x0000d2 R E 0x1000 >>>>> LOAD 0x004000 0x20000 0x20000 0x000004 0x000004 RW 0x1000 >>>>> >>>>> Old kernels load this ELF file in the following way ('/proc/self/maps'): >>>>> >>>>> 00010000-00011000 r--p 00001000 00:02 131 ./bug-reproduction >>>>> 00011000-00012000 rw-p 00002000 00:02 131 ./bug-reproduction >>>>> 00012000-00013000 r-xp 00003000 00:02 131 ./bug-reproduction >>>>> 00020000-00021000 rw-p 00004000 00:02 131 ./bug-reproduction >>>>> >>>>> And new kernels do it like this: >>>>> >>>>> 00010000-00011000 r--p 00001000 00:02 131 ./bug-reproduction >>>>> 00011000-00012000 rw-p 00000000 00:00 0 >>>>> 00012000-00013000 r-xp 00003000 00:02 131 ./bug-reproduction >>>>> 00020000-00021000 rw-p 00004000 00:02 131 ./bug-reproduction >>>>> >>>>> That map between 0x11000 and 0x12000 is the program's '.data' and '.bss' >>>>> sections to which it tries to write to, and since the kernel doesn't map >>>>> them anymore it crashes. >>>>> >>>>> I bisected the issue to the following commit: >>>>> >>>>> commit 585a018627b4d7ed37387211f667916840b5c5ea >>>>> Author: Eric W. Biederman <ebiederm@xxxxxxxxxxxx> >>>>> Date: Thu Sep 28 20:24:29 2023 -0700 >>>>> >>>>> binfmt_elf: Support segments with 0 filesz and misaligned starts >>>>> >>>>> I can confirm that with this commit the issue reproduces, and with it >>>>> reverted it doesn't. >>>>> >>>>> I have prepared a minimal reproduction of the problem available here, >>>>> along with all of the scripts I used for bisecting: >>>>> >>>>> https://github.com/koute/linux-elf-loading-bug >>>>> >>>>> You can either compile it from source (requires Rust and LLD), or there's >>>>> a prebuilt binary in 'bin/bug-reproduction` which you can run. (It's tiny, >>>>> so you can easily check with 'objdump -d' that it isn't malicious). >>>>> >>>>> On old kernels this will run fine, and on new kernels it will >>>>> segfault. >>>> >>>> Frankly your ELF binary is buggy, and probably the best fix would be to >>>> fix the linker script that is used to generate your binary. >>>> >>>> The problem is the SYSV ABI defines everything in terms of pages and so >>>> placing two ELF segments on the same page results in undefined behavior. >>>> >>>> The code was fixed to honor your .bss segment and now your .data segment >>>> is being stomped, because you defined them to overlap. >>>> >>>> Ideally your linker script would place both your .data and .bss in >>>> the same segment. That would both fix the issue and give you a more >>>> compact elf binary, while not changing the generated code at all. >>>> >>>> >>>> That said regressions suck and it would be good if we could update the >>>> code to do something reasonable in this case. >>>> >>>> We can perhaps we can update the .bss segment to just memset an existing >>>> page if one has already been mapped. Which would cleanly handle a case >>>> like yours. I need to think about that for a moment to see what the >>>> code would look like to do that. >>> >>> It's the "if one has already been mapped" part which might >>> become expensive... >> >> I am wondering if perhaps we can add MAP_FIXED_NOREPLACE and take >> some appropriate action if there is already a mapping there. > > Yeah, in the general case we had to back out MAP_FIXED_NOREPLACE usage > for individual LOADs because there were so many cases of overlapping > LOADs. :( Currently it's only used during the initial mapping (when > "total_size" is set), to avoid colliding with the stack. > > But, as you suggest, if we only use it for filesz==0, it could work. > >> Such as printing a warning and skipping the action entirely for >> a pure bss segment. That would essentially replicate the previous >> behavior. > > Instead of failing, perhaps we just fallback to not using > MAP_FIXED_NOREPLACE and do the memset? (And maybe pr_warn_once?) > >> At a minimum adding MAP_FIXED_NOREPLACE should allow us to >> deterministically detect and warn about problems, making it easier >> for people to understand why their binary won't run. > > Yeah, it seems like it's the vm_brk_flags() that is clobber the mapping, > so we have to skip that for the MAP_FIXED_NOREPLACE fails on a filesz==0 > case? >
