5.4-stable review patch. If anyone has any objections, please let me know.
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From: David Disseldorp ddiss@suse.de
[ Upstream commit e017671f534dd3f568db9e47b0583e853d2da9b5 ]
The initramfs filename field is defined in Documentation/driver-api/early-userspace/buffer-format.rst as:
37 cpio_file := ALGN(4) + cpio_header + filename + "\0" + ALGN(4) + data ... 55 ============= ================== ========================= 56 Field name Field size Meaning 57 ============= ================== ========================= ... 70 c_namesize 8 bytes Length of filename, including final \0
When extracting an initramfs cpio archive, the kernel's do_name() path handler assumes a zero-terminated path at @collected, passing it directly to filp_open() / init_mkdir() / init_mknod().
If a specially crafted cpio entry carries a non-zero-terminated filename and is followed by uninitialized memory, then a file may be created with trailing characters that represent the uninitialized memory. The ability to create an initramfs entry would imply already having full control of the system, so the buffer overrun shouldn't be considered a security vulnerability.
Append the output of the following bash script to an existing initramfs and observe any created /initramfs_test_fname_overrunAA* path. E.g. ./reproducer.sh | gzip >> /myinitramfs
It's easiest to observe non-zero uninitialized memory when the output is gzipped, as it'll overflow the heap allocated @out_buf in __gunzip(), rather than the initrd_start+initrd_size block.
---- reproducer.sh ---- nilchar="A" # change to "\0" to properly zero terminate / pad magic="070701" ino=1 mode=$(( 0100777 )) uid=0 gid=0 nlink=1 mtime=1 filesize=0 devmajor=0 devminor=1 rdevmajor=0 rdevminor=0 csum=0 fname="initramfs_test_fname_overrun" namelen=$(( ${#fname} + 1 )) # plus one to account for terminator
printf "%s%08x%08x%08x%08x%08x%08x%08x%08x%08x%08x%08x%08x%08x%s" \ $magic $ino $mode $uid $gid $nlink $mtime $filesize \ $devmajor $devminor $rdevmajor $rdevminor $namelen $csum $fname
termpadlen=$(( 1 + ((4 - ((110 + $namelen) & 3)) % 4) )) printf "%.s${nilchar}" $(seq 1 $termpadlen) ---- reproducer.sh ----
Symlink filename fields handled in do_symlink() won't overrun past the data segment, due to the explicit zero-termination of the symlink target.
Fix filename buffer overrun by aborting the initramfs FSM if any cpio entry doesn't carry a zero-terminator at the expected (name_len - 1) offset.
Fixes: 1da177e4c3f41 ("Linux-2.6.12-rc2") Signed-off-by: David Disseldorp ddiss@suse.de Link: https://lore.kernel.org/r/20241030035509.20194-2-ddiss@suse.de Signed-off-by: Christian Brauner brauner@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org --- init/initramfs.c | 15 +++++++++++++++ 1 file changed, 15 insertions(+)
diff --git a/init/initramfs.c b/init/initramfs.c index b362b57c047d5..71ca68f403993 100644 --- a/init/initramfs.c +++ b/init/initramfs.c @@ -323,6 +323,15 @@ static int __init do_name(void) { state = SkipIt; next_state = Reset; + + /* name_len > 0 && name_len <= PATH_MAX checked in do_header */ + if (collected[name_len - 1] != '\0') { + pr_err("initramfs name without nulterm: %.*s\n", + (int)name_len, collected); + error("malformed archive"); + return 1; + } + if (strcmp(collected, "TRAILER!!!") == 0) { free_hash(); return 0; @@ -385,6 +394,12 @@ static int __init do_copy(void)
static int __init do_symlink(void) { + if (collected[name_len - 1] != '\0') { + pr_err("initramfs symlink without nulterm: %.*s\n", + (int)name_len, collected); + error("malformed archive"); + return 1; + } collected[N_ALIGN(name_len) + body_len] = '\0'; clean_path(collected, 0); ksys_symlink(collected + N_ALIGN(name_len), collected);