Kees Cook <keescook@xxxxxxxxxxxx> writes: > On Tue, Feb 27, 2024 at 09:35:39AM -0600, Eric W. Biederman wrote: >> Kees Cook <keescook@xxxxxxxxxxxx> writes: >> >> > On Tue, Jan 23, 2024 at 12:23:27AM +0900, Jan Bujak wrote: >> >> On 1/22/24 23:54, Pedro Falcato wrote: >> >> > Hi! >> >> > >> >> > Where did you get that linker script? >> >> > >> >> > FWIW, I catched this possible issue in review, and this was already >> >> > discussed (see my email and Eric's reply): >> >> > https://lore.kernel.org/all/CAKbZUD3E2if8Sncy+M2YKncc_Zh08-86W6U5wR0ZMazShxbHHA@xxxxxxxxxxxxxx/ >> >> > >> >> > This was my original testcase >> >> > (https://github.com/heatd/elf-bug-questionmark), which convinced the >> >> > loader to map .data over a cleared .bss. Your bug seems similar, but >> >> > does the inverse: maps .bss over .data. >> >> > >> >> >> >> I wrote the linker script myself from scratch. >> > >> > Do you still need this addressed, or have you been able to adjust the >> > linker script? (I ask to try to assess the priority of needing to fix >> > this behavior change...) >> >> Kees, I haven't had a chance to test this yet but it occurred to me >> that there is an easy way to handle this. In our in-memory copy >> of the elf program headers we can just merge the two segments >> together. >> >> I believe the diff below accomplishes that, and should fix issue. >> >> Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx> >> >> diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c >> index 5397b552fbeb..01df7dd1f3b4 100644 >> --- a/fs/binfmt_elf.c >> +++ b/fs/binfmt_elf.c >> @@ -924,6 +926,31 @@ static int load_elf_binary(struct linux_binprm *bprm) >> elf_ppnt = elf_phdata; >> for (i = 0; i < elf_ex->e_phnum; i++, elf_ppnt++) >> switch (elf_ppnt->p_type) { >> + case PT_LOAD: >> + { >> + /* >> + * Historically linux ignored all but the >> + * final .bss segment. Now that linux honors >> + * all .bss segments, a .bss segment that >> + * logically is not overlapping but is >> + * overlapping when it's edges are rounded up >> + * to page size causes programs to fail. >> + * >> + * Handle that case by merging .bss segments >> + * into the segment they follow. >> + */ >> + if (((i + 1) >= elf_ex->e_phnum) || >> + (elf_ppnt[1].p_type != PT_LOAD) || >> + (elf_ppnt[1].p_filesz != 0)) >> + continue; >> + unsigned long end = >> + elf_ppnt[0].p_vaddr + elf_ppnt[0].p_memsz; >> + if (elf_ppnt[1].p_vaddr != end) >> + continue; >> + elf_ppnt[0].p_memsz += elf_ppnt[1].p_memsz; >> + elf_ppnt[1].p_type = PT_NULL; >> + break; >> + } >> case PT_GNU_STACK: >> if (elf_ppnt->p_flags & PF_X) >> executable_stack = EXSTACK_ENABLE_X; > > I don't think this is safe -- it isn't looking at flags, etc. e.g., > something like this could break: > > Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align > LOAD 0x003000 0x12000 0x12000 0x001000 0x001000 R E 0x1000 > LOAD 0x004000 0x13000 0x13000 0x000000 0x001000 RW 0x1000 Yes. I think it should be modified to only do something is the break is not on a page boundary (which will automatically limit it's effect to where we need to do something for backwards compatibility). Still with a few tweaks and testing I think that is a good path forward for dealing with the ``regression'' case. Eric
