Hello all,
I raised this patch on #linuxfs on IRC, and I got asked if this actually
effects real programs. To demonstrate that it does, I wrote up a simple
C program which just does a table lookup of a prime number. The table is
stored sparsely, so newer versions of GCC+LD automatically put each
table entry in its own program section and segment. This results in over
100 ELF program header entries, which Linux since 3.19 will refuse to
load with ENOEXEC due to the errant limit fixed in my patch. (The
current broken limit is 73, whereas the manpage states a limit of 64k.)
My example program is available at
https://www.cs.unc.edu/~jbakita/get_prime.c and should be built as gcc
get_prime.c -o get_prime. I know this works with GCC 9.3.0 and LD 2.34
(GCC 7.5.0 and LD 2.30 are too old). You can verify it built correctly
by checking the "Number of program headers" as printed by readelf -h is
at least 100.
I tried to keep this patch small to make it easy to review, but there
are a few other bugs (like the 64KB limit) in the ELF loader. Would it
be more helpful or make review easier to just fix all the bugs at once?
This is my first kernel patch, and I'd really like to make it the first
of many.
Best,
Joshua Bakita
On 12/14/20 10:46 PM, Joshua Bakita wrote:
Commit 6a8d38945cf4 ("binfmt_elf: Hoist ELF program header loading to a
function") merged load_elf_binary and load_elf_interp into
load_elf_phdrs. This change imposed a limit that the program headers of
all ELF binaries are smaller than ELF_MIN_ALIGN. This is a mistake for
two reasons:
1. load_elf_binary previously had no such constraint, meaning that
previously valid ELF program headers are now rejected by the kernel as
oversize and invalid.
2. The ELF interpreter's program headers should never have been limited to
ELF_MIN_ALIGN (and previously PAGE_SIZE) in the first place. Commit
057f54fbba73 ("Import 1.1.54") introduced this limit to the ELF
interpreter alongside the initial ELF parsing support without any
explanation.
This patch removes the ELF_MIN_ALIGN size constraint in favor of only
relying on an earlier check that the allocation will be less than 64KiB.
(It's worth mentioning that the 64KiB limit is also unnecessarily strict,
but that's not addressed here for simplicity. The ELF manpage says that
the program header size is supposed to have at most 64 thousand entries,
not less than 64 thousand bytes.)
Fixes: 6a8d38945cf4 ("binfmt_elf: Hoist ELF program header loading to a function")
Signed-off-by: Joshua Bakita <jbakita@xxxxxxxxxx>
---
fs/binfmt_elf.c | 4 ----
1 file changed, 4 deletions(-)
diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index 2472af2798c7..55162056590f 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -412,15 +412,11 @@ static struct elf_phdr *load_elf_phdrs(struct elfhdr *elf_ex,
/* Sanity check the number of program headers... */
if (elf_ex->e_phnum < 1 ||
elf_ex->e_phnum > 65536U / sizeof(struct elf_phdr))
goto out;
- /* ...and their total size. */
size = sizeof(struct elf_phdr) * elf_ex->e_phnum;
- if (size > ELF_MIN_ALIGN)
- goto out;
-
elf_phdata = kmalloc(size, GFP_KERNEL);
if (!elf_phdata)
goto out;
/* Read in the program headers */
