Re: Recent-ish changes in binfmt_elf made my program segfault

["Eric W. Biederman" <ebiederm@xmission.com>]

Kees Cook <keescook@chromium.org> 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@mail.gmail.com/
>> > 
>> > 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@xmission.com>

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;