FreeBSD Security Advisory FreeBSD-SA-21:11.smap

Thu May 27 17:35:58 UTC 2021

Since I had a question on this in another forum, I figure I'll copy it to the public list as well. The credit line below was specifically requested by the reporter. It wasn't a typo or a lack of proof-reading on our part.

Best,
Gordon
Hat: security-officer

> On May 26, 2021, at 5:54 PM, FreeBSD Security Advisories <security-advisories at freebsd.org> wrote:
> 
> Signed PGP part
> =============================================================================
> FreeBSD-SA-21:11.smap                                       Security Advisory
>                                                           The FreeBSD Project
> 
> Topic:          SMAP bypass
> 
> Category:       core
> Module:         amd64
> Announced:      2021-05-26
> Credits:        I lost my dog if you see him please contact me at @m00nbsd.
> Affects:        FreeBSD 12.2 and later.
> Corrected:      2021-05-26 19:18:54 UTC (stable/13, 13.0-STABLE)
>                 2021-05-26 19:31:50 UTC (releng/13.0, 13.0-RELEASE-p1)
>                 2021-05-26 19:30:31 UTC (stable/12, 12.2-STABLE)
>                 2021-05-26 20:40:20 UTC (releng/12.2, 12.2-RELEASE-p7)
> CVE Name:       CVE-2021-29628
> 
> For general information regarding FreeBSD Security Advisories,
> including descriptions of the fields above, security branches, and the
> following sections, please visit <URL:https://security.FreeBSD.org/>.
> 
> I.   Background
> 
> Supervisor Mode Access Prevention (SMAP) is a security feature
> implemented by contemporary Intel and AMD CPUs.  When enabled, it
> ensures that accesses to user memory by the kernel trigger a page fault
> and a subsequent kernel panic.  This helps mitigate the security
> implications of kernel bugs that permit an attacker to read from or
> write to user memory from the kernel.
> 
> The kernel may legitimately need to copy data between userspace and the
> kernel.  To enable this, SMAP is temporarily disabled in the subroutines
> which handle this copying, so only small, specially designated portions
> of the kernel should be executed with SMAP disabled.
> 
> II.  Problem Description
> 
> The FreeBSD kernel enables SMAP during boot when the CPU reports that
> the SMAP capability is present.  Subroutines such as copyin() and
> copyout() are responsible for disabling SMAP around the sections of code
> that perform user memory accesses.
> 
> Such subroutines must handle page faults triggered when user memory is
> not mapped.  The kernel's page fault handler checks the validity of the
> fault, and if it is indeed valid it will map a page and resume copying.
> If the fault is invalid, the fault handler returns control to a
> trampoline which aborts the operation and causes an error to be
> returned.  In this second scenario, a bug in the implementation of SMAP
> support meant that SMAP would remain disabled until the thread returns
> to user mode.
> 
> III. Impact
> 
> This bug may be used to bypass the protections provided by SMAP for the
> duration of a system call.  It could thus be combined with other kernel
> bugs to craft an exploit.
> 
> IV.  Workaround
> 
> No workaround is available.  On hardware that does not implement SMAP,
> the bug is inconsequential as the mitigation does not exist in the first
> place.
> 
> V.   Solution
> 
> Upgrade your vulnerable system to a supported FreeBSD stable or
> release / security branch (releng) dated after the correction date
> and reboot.
> 
> Perform one of the following:
> 
> 1) To update your vulnerable system via a binary patch:
> 
> Systems running a RELEASE version of FreeBSD on the amd64, i386, or
> (on FreeBSD 13 and later) arm64 platforms can be updated via the
> freebsd-update(8) utility:
> 
> # freebsd-update fetch
> # freebsd-update install
> # shutdown -r +10min "Rebooting for a security update"
> 
> 2) To update your vulnerable system via a source code patch:
> 
> The following patches have been verified to apply to the applicable
> FreeBSD release branches.
> 
> a) Download the relevant patch from the location below, and verify the
> detached PGP signature using your PGP utility.
> 
> # fetch https://security.FreeBSD.org/patches/SA-21:11/smap.patch
> # fetch https://security.FreeBSD.org/patches/SA-21:11/smap.patch.asc
> # gpg --verify smap.patch.asc
> 
> b) Apply the patch.  Execute the following commands as root:
> 
> # cd /usr/src
> # patch < /path/to/patch
> 
> c) Recompile your kernel as described in
> <URL:https://www.FreeBSD.org/handbook/kernelconfig.html> and reboot the
> system.
> 
> VI.  Correction details
> 
> This issue is corrected by the corresponding Git commit hash or Subversion
> revision number in the following stable and release branches:
> 
> Branch/path                             Hash                     Revision
> -------------------------------------------------------------------------
> stable/13/                              876ffe28796c    stable/13-n245764
> releng/13.0/                            f32130a1955e  releng/13.0-n244739
> stable/12/                                                        r369857
> releng/12.2/                                                      r369863
> -------------------------------------------------------------------------
> 
> For FreeBSD 13 and later:
> 
> Run the following command to see which files were modified by a
> particular commit:
> 
> # git show --stat <commit hash>
> 
> Or visit the following URL, replacing NNNNNN with the hash:
> 
> <URL:https://cgit.freebsd.org/src/commit/?id=NNNNNN>
> 
> To determine the commit count in a working tree (for comparison against
> nNNNNNN in the table above), run:
> 
> # git rev-list --count --first-parent HEAD
> 
> For FreeBSD 12 and earlier:
> 
> Run the following command to see which files were modified by a particular
> revision, replacing NNNNNN with the revision number:
> 
> # svn diff -cNNNNNN --summarize svn://svn.freebsd.org/base
> 
> Or visit the following URL, replacing NNNNNN with the revision number:
> 
> <URL:https://svnweb.freebsd.org/base?view=revision&revision=NNNNNN>
> 
> VII. References
> 
> <URL:https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2021-29628>
> 
> The latest revision of this advisory is available at
> <URL:https://security.FreeBSD.org/advisories/FreeBSD-SA-21:11.smap.asc>
> 
> 

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