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Message-ID: <CAGXu5jJw9dDOQRwLCGXhTjii6tZkLC2C+O3MV8tBoh3OV9mFNQ@mail.gmail.com> Date: Tue, 29 Sep 2015 10:47:45 -0700 From: Kees Cook <keescook@...omium.org> To: Andy Lutomirski <luto@...nel.org> Cc: Michael Kerrisk-manpages <mtk.manpages@...il.com>, Serge Hallyn <serge.hallyn@...ntu.com>, Andrew Morton <akpm@...uxfoundation.org>, Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>, "Ted Ts'o" <tytso@....edu>, "Andrew G. Morgan" <morgan@...nel.org>, Linux API <linux-api@...r.kernel.org>, Mimi Zohar <zohar@...ux.vnet.ibm.com>, Austin S Hemmelgarn <ahferroin7@...il.com>, linux-security-module <linux-security-module@...r.kernel.org>, Aaron Jones <aaronmdjones@...il.com>, Serge Hallyn <serge.hallyn@...onical.com>, LKML <linux-kernel@...r.kernel.org>, Markku Savela <msa@...h.iki.fi>, Jonathan Corbet <corbet@....net> Subject: Re: [PATCH v2] capabilities.7, prctl.2: Document ambient capabilities On Mon, Sep 28, 2015 at 6:03 PM, Andy Lutomirski <luto@...nel.org> wrote: > Signed-off-by: Andy Lutomirski <luto@...nel.org> > --- > man2/prctl.2 | 12 ++++++++++++ > man7/capabilities.7 | 40 ++++++++++++++++++++++++++++++++++------ > 2 files changed, 46 insertions(+), 6 deletions(-) > > diff --git a/man2/prctl.2 b/man2/prctl.2 > index e743a6305969..5bcec391c110 100644 > --- a/man2/prctl.2 > +++ b/man2/prctl.2 > @@ -954,6 +954,18 @@ had been called. > For further information on Intel MPX, see the kernel source file > .IR Documentation/x86/intel_mpx.txt . > .\" > +.TP > +.BR PR_CAP_AMBIENT " (since Linux 4.2)" > +Reads or changes the ambient capability set. If arg2 is PR_CAP_AMBIENT_RAISE, > +then the capability specified in arg3 is added to the ambient set. This will > +fail, returning EPERM, if the capability is not already both permitted and > +inheritable or if the SECBIT_NO_CAP_AMBIENT_RAISE securebit is set. If arg2 > +is PR_CAP_AMBIENT_LOWER, then the capability specified in arg3 is removed > +from the ambient set. If arg2 is PR_CAP_AMBIENT_IS_SET, then > +.BR prctl (2) > +will return 1 if the capability in arg3 is in the ambient set and 0 if not. > +If arg2 is PR_CAP_AMBIENT_CLEAR_ALL, then all capabilities will > +be removed from the ambient set. In the case of CLEAR_ALL, is arg3 "don't care", or must it be set to 0? > .SH RETURN VALUE > On success, > .BR PR_GET_DUMPABLE , > diff --git a/man7/capabilities.7 b/man7/capabilities.7 > index 616189c881e4..8934d05a5b07 100644 > --- a/man7/capabilities.7 > +++ b/man7/capabilities.7 > @@ -700,13 +700,34 @@ a program whose associated file capabilities grant that capability). > .IR Inheritable : > This is a set of capabilities preserved across an > .BR execve (2). > -It provides a mechanism for a process to assign capabilities > -to the permitted set of the new program during an > -.BR execve (2). > +Inheritable capabilities remain inheritable when executing any program, > +and inheritable capabilities are added to the permitted set when executing > +a program that has the corresponding bits set in the file inheritable set. > +.IP > +Because inheritable capabilities are not generally preserved across > +.BR execve (2) > +when running as a non-root user, applications that wish to run helper > +programs with elevated capabilities should consider using ambient capabilities, > +described below. > .TP > .IR Effective : > This is the set of capabilities used by the kernel to > perform permission checks for the thread. > +.TP > +.IR Ambient " (since Linux 4.3):" > +This is a set of capabilities that are preserved across an > +.BR execve (2) > +of a program that does not have file capabilities. The ambient capability > +set obeys the invariant that no capability can ever be ambient if it is > +not both permitted and inheritable. Ambient capabilities are > +preserved in the permitted set and added to the effective > +set when > +.BR execve (2) > +is called. The ambient capability set is modified using > +.BR prctl (2). > +Executing a program that changes uid or gid due to the setuid or setgid > +bits or executing a program that has any file capabilities set will clear > +the ambient set. > .PP > A child created via > .BR fork (2) > @@ -788,10 +809,12 @@ the process using the following algorithm: > .in +4n > .nf > > +P'(ambient) = (file has capabilities or is setuid or setgid) ? 0 : P(ambient) > + > P'(permitted) = (P(inheritable) & F(inheritable)) | > - (F(permitted) & cap_bset) > + (F(permitted) & cap_bset) | P'(ambient) > > -P'(effective) = F(effective) ? P'(permitted) : 0 > +P'(effective) = F(effective) ? P'(permitted) : P'(ambient) > > P'(inheritable) = P(inheritable) [i.e., unchanged] > > @@ -1074,6 +1097,10 @@ an effective or real UID of 0 calls > .BR execve (2). > (See the subsection > .IR "Capabilities and execution of programs by root" .) > +.TP > +.B SECBIT_NO_CAP_AMBIENT_RAISE > +Setting this flag disallows > +.BR PR_CAP_AMBIENT_RAISE . > .PP > Each of the above "base" flags has a companion "locked" flag. > Setting any of the "locked" flags is irreversible, > @@ -1082,8 +1109,9 @@ corresponding "base" flag. > The locked flags are: > .BR SECBIT_KEEP_CAPS_LOCKED , > .BR SECBIT_NO_SETUID_FIXUP_LOCKED , > +.BR SECBIT_NOROOT_LOCKED , > and > -.BR SECBIT_NOROOT_LOCKED . > +.BR SECBIT_NO_CAP_AMBIENT_RAISE . > .PP > The > .I securebits > -- > 2.4.3 > Looks great, thanks! Reviewed-by: Kees Cook <keescook@...omium.org> -Kees -- Kees Cook Chrome OS Security -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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