CVE-2026-68186: binfmt_misc: set have_execfd only once the interpreter is opened

From: Greg Kroah-Hartman <[email protected]>

Description
===========

In the Linux kernel, the following vulnerability has been resolved:

binfmt_misc: set have_execfd only once the interpreter is opened

load_misc_binary() raises bprm->have_execfd as soon as it sees the 'O'
(or 'C') flag. This happens well before it opens the interpreter. If
that open fails the flag stays set on the bprm. binfmt_misc is at the
head of the format list so an interpreter open failure that returns
-ENOEXEC lets the search fall through to a later format. This means it
runs the matched binary directly having never staged an interpreter. So
bprm->executable is NULL while have_execfd falsely claims a descriptor
is present.

Consequently, begin_new_exec() dereferences the missing executable:

  would_dump(bprm, bprm->executable);

and NULL derefs. Had it not, the hand-off later in the same function
would have failed anyway. FD_ADD(0, bprm->executable) rejects a NULL
file with -ENOMEM. Both sites are past the point of no return so the
exec cannot be unwound either way.

This can be reached by unprivileged users as binfmt_misc can be mounted
in user namespaces. So a user can register an 'O' entry whose
interpreter lives on a FUSE mount, have the FUSE server fail the open
with -ENOEXEC and execute a native ELF file that matches the entry.

have_execfd only means anything alongside the executable it describes
which is not set until the interpreter has been opened and staged.
So lets raise it there, next to execfd_creds, which is already set at
that point. An open failure now leaves it clear, so the fallback format
derives credentials from the binary and emits no AT_EXECFD, as it would
for any native exec. The argv rewrite load_misc_binary() performs before
the open is still not undone. This means the binary sees the interpreter
path in argv[0] and its own path in argv[1] but that predates this
change and only became observable once the exec stopped faulting.

The Linux kernel CVE team has assigned CVE-2026-68186 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 5.8 with commit bc2bf338d54b7aadaed49bb45b9e10d4592b2a46 and fixed in 6.6.148 with commit 0f19d54e2524f0bf183b82f365ae4e49b4a2f788
	Issue introduced in 5.8 with commit bc2bf338d54b7aadaed49bb45b9e10d4592b2a46 and fixed in 6.12.101 with commit 2dd0298905e97795a9c5ec30cf5b41975f821632
	Issue introduced in 5.8 with commit bc2bf338d54b7aadaed49bb45b9e10d4592b2a46 and fixed in 6.18.42 with commit 1cd4e9b7967dab48c9f79a00b06ffff7208c0993
	Issue introduced in 5.8 with commit bc2bf338d54b7aadaed49bb45b9e10d4592b2a46 and fixed in 7.1.6 with commit 5ccc99d58f94fad258c9c375715b3974e48620e8
	Issue introduced in 5.8 with commit bc2bf338d54b7aadaed49bb45b9e10d4592b2a46 and fixed in 7.2-rc5 with commit bbf5f639918dc011aaf60aab8480218758ee68c5

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-68186
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	fs/binfmt_misc.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/0f19d54e2524f0bf183b82f365ae4e49b4a2f788
	https://git.kernel.org/stable/c/2dd0298905e97795a9c5ec30cf5b41975f821632
	https://git.kernel.org/stable/c/1cd4e9b7967dab48c9f79a00b06ffff7208c0993
	https://git.kernel.org/stable/c/5ccc99d58f94fad258c9c375715b3974e48620e8
	https://git.kernel.org/stable/c/bbf5f639918dc011aaf60aab8480218758ee68c5