Messages in this thread | | | Date | Wed, 12 Aug 2020 18:00:17 +0200 | From | Jessica Yu <> | Subject | Re: [PATCH v2] module: Harden STRICT_MODULE_RWX |
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+++ Szabolcs Nagy [12/08/20 15:15 +0100]: >The 08/12/2020 13:56, Will Deacon wrote: >> On Wed, Aug 12, 2020 at 12:40:05PM +0200, peterz@infradead.org wrote: >> > On Wed, Aug 12, 2020 at 10:56:56AM +0200, Ard Biesheuvel wrote: >> > > The module .lds has BYTE(0) in the section contents to prevent the >> > > linker from pruning them entirely. The (NOLOAD) is there to ensure >> > > that this byte does not end up in the .ko, which is more a matter of >> > > principle than anything else, so we can happily drop that if it helps. >> > > >> > > However, this should only affect the PROGBITS vs NOBITS designation, >> > > and so I am not sure whether it makes a difference. >> > > >> > > Depending on where the w^x check occurs, we might simply override the >> > > permissions of these sections, and strip the writable permission if it >> > > is set in the PLT handling init code, which manipulates the metadata >> > > of all these 3 sections before the module space is vmalloc'ed. >> > >> > What's curious is that this seems the result of some recent binutils >> > change. Every build with binutils-2.34 (or older) does not seem to >> > generate these as WAX, but has the much more sensible WA. >> > >> > I suppose we can change the kernel check and 'allow' W^X for 0 sized >> > sections, but I think we should still figure out why binutils-2.35 is >> > now generating WAX sections all of a sudden, it might come bite us >> > elsewhere. >> >> Agreed, I think it's important to figure out what's going on here before we >> try to bodge around it. >> >> Adding Szabolcs, in case he has any ideas. >> >> To save him reading the whole thread, here's a summary: >> >> AArch64 kernel modules built with binutils 2.35 end up with a couple of >> ELF sections marked as SHF_WRITE | SHF_ALLOC | SHF_EXECINSTR: >> >> [ 5] .plt PROGBITS 0000000000000388 01d000 000008 00 WAX 0 0 1 >> [ 6] .init.plt NOBITS 0000000000000390 01d008 000008 00 WA 0 0 1 >> [ 7] .text.ftrace_trampoline PROGBITS 0000000000000398 01d008 000008 00 WAX 0 0 1 >> >> This results in the module being rejected by our loader, because we don't >> permit writable, executable mappings. >> >> Our linker script for these entries uses NOLOAD, so it's odd to see PROGBITS >> appearing in the readelf output above (and older binutils emits NOBITS >> sections). Anyway, here's the linker script: >> >> SECTIONS { >> .plt (NOLOAD) : { BYTE(0) } >> .init.plt (NOLOAD) : { BYTE(0) } >> .text.ftrace_trampoline (NOLOAD) : { BYTE(0) } >> } >> >> It appears that the name of the section influences the behaviour, as >> Jessica observed [1] that sections named .text.* end up with PROGBITS, >> whereas random naming such as ".test" ends up with NOBITS, as before. >> >> We've looked at the changelog between binutils 2.34 and 2.35, but nothing >> stands out. Any clues? Is this intentional binutils behaviour? > >for me it bisects to > >https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=8c803a2dd7d3d742a3d0071914f557ef465afe71 > >i will have to investigate further what's going on.
Thanks for the hint. I'm almost certain it's due to this excerpt from the changelog: "we now init sh_type and sh_flags for all known ABI sections in _bfd_elf_new_section_hook."
Indeed, .plt and .text.* are listed as special sections in bfd/elf.c. The former requires an exact match and the latter only has to match the prefix ".text." Since the code considers ".plt" and ".text.ftrace_trampoline" special sections, their sh_type and sh_flags are now set by default. Now I guess the question is whether this can be overriden by a linker script..
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