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[2003:c9:3f1e:c600:a890:fb4d:6849:2d4]) by smtp.gmail.com with ESMTPSA id vr10-20020a05620a55aa00b0076ef7810f27sm3633710qkn.58.2023.09.25.02.20.53 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 25 Sep 2023 02:20:54 -0700 (PDT) Date: Mon, 25 Sep 2023 11:20:51 +0200 (CEST) From: Sebastian Ott To: "Eric W. Biederman" cc: =?ISO-8859-15?Q?Thomas_Wei=DFschuh?= , Alexander Viro , Christian Brauner , Kees Cook , Mark Brown , Willy Tarreau , linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org Subject: Re: [PATCH RFC] binfmt_elf: fully allocate bss pages In-Reply-To: <87zg1bm5xo.fsf@email.froward.int.ebiederm.org> Message-ID: <37d3392c-cf33-20a6-b5c9-8b3fb8142658@redhat.com> References: <20230914-bss-alloc-v1-1-78de67d2c6dd@weissschuh.net> <36e93c8e-4384-b269-be78-479ccc7817b1@redhat.com> <87zg1bm5xo.fsf@email.froward.int.ebiederm.org> MIME-Version: 1.0 Content-Type: text/plain; format=flowed; charset=US-ASCII Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org On Sun, 24 Sep 2023, Eric W. Biederman wrote: > Sebastian Ott writes: > >> Hej, >> >> since we figured that the proposed patch is not going to work I've spent a >> couple more hours looking at this (some static binaries on arm64 segfault >> during load [0]). The segfault happens because of a failed clear_user() >> call in load_elf_binary(). The address we try to write zeros to is mapped with >> correct permissions. >> >> After some experiments I've noticed that writing to anonymous mappings work >> fine and all the error cases happend on file backed VMAs. Debugging showed that >> in elf_map() we call vm_mmap() with a file offset of 15 pages - for a binary >> that's less than 1KiB in size. >> >> Looking at the ELF headers again that 15 pages offset originates from the offset >> of the 2nd segment - so, I guess the loader did as instructed and that binary is >> just too nasty? >> >> Program Headers: >> Type Offset VirtAddr PhysAddr >> FileSiz MemSiz Flags Align >> LOAD 0x0000000000000000 0x0000000000400000 0x0000000000400000 >> 0x0000000000000178 0x0000000000000178 R E 0x10000 >> LOAD 0x000000000000ffe8 0x000000000041ffe8 0x000000000041ffe8 >> 0x0000000000000000 0x0000000000000008 RW 0x10000 >> NOTE 0x0000000000000120 0x0000000000400120 0x0000000000400120 >> 0x0000000000000024 0x0000000000000024 R 0x4 >> GNU_STACK 0x0000000000000000 0x0000000000000000 0x0000000000000000 >> 0x0000000000000000 0x0000000000000000 RW 0x10 >> >> As an additional test I've added a bunch of zeros at the end of that binary >> so that the offset is within that file and it did load just fine. >> >> On the other hand there is this section header: >> [ 4] .bss NOBITS 000000000041ffe8 0000ffe8 >> 0000000000000008 0000000000000000 WA 0 0 1 >> >> "sh_offset >> This member's value gives the byte offset from the beginning of the file to >> the first byte in the section. One section type, SHT_NOBITS described >> below, occupies no space in the file, and its sh_offset member locates >> the conceptual placement in the file. >> " >> >> So, still not sure what to do here.. >> >> Sebastian >> >> [0] https://lore.kernel.org/lkml/5d49767a-fbdc-fbe7-5fb2-d99ece3168cb@redhat.com/ > > I think that .bss section that is being generated is atrocious. > > At the same time I looked at what the linux elf loader is trying to do, > and the elf loader's handling of program segments with memsz > filesz > has serious remnants a.out of programs allocating memory with the brk > syscall. > > Lots of the structure looks like it started with the assumption that > there would only be a single program header with memsz > filesz the way > and that was the .bss. The way things were in the a.out days and > handling of other cases has been debugged in later. > > So I have modified elf_map to always return successfully when there is > a zero filesz in the program header for an elf segment. > > Then I have factored out a function clear_tail that ensures the zero > padding for an entire elf segment is present. > > Please test this and see if it causes your test case to work. Sadly, that causes issues for other programs: [ 44.164596] Run /init as init process [ 44.168763] Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000000b [ 44.176409] CPU: 32 PID: 1 Comm: init Not tainted 6.6.0-rc2+ #89 [ 44.182404] Hardware name: GIGABYTE R181-T92-00/MT91-FS4-00, BIOS F34 08/13/2020 [ 44.189786] Call trace: [ 44.192220] dump_backtrace+0xa4/0x130 [ 44.195961] show_stack+0x20/0x38 [ 44.199264] dump_stack_lvl+0x48/0x60 [ 44.202917] dump_stack+0x18/0x28 [ 44.206219] panic+0x2e0/0x350 [ 44.209264] do_exit+0x370/0x390 [ 44.212481] do_group_exit+0x3c/0xa0 [ 44.216044] get_signal+0x800/0x808 [ 44.219521] do_signal+0xfc/0x200 [ 44.222824] do_notify_resume+0xc8/0x418 [ 44.226734] el0_da+0x114/0x120 [ 44.229866] el0t_64_sync_handler+0xb8/0x130 [ 44.234124] el0t_64_sync+0x194/0x198 [ 44.237776] SMP: stopping secondary CPUs [ 44.241740] Kernel Offset: disabled [ 44.245215] CPU features: 0x03000000,14028142,10004203 [ 44.250342] Memory Limit: none [ 44.253383] ---[ end Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000000b ]---