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use a smaller subset of flags for remounting readonly

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a) In the linux namespace mount case an additional MS_NOATIME, etc. will result in
EPERM on remounting something hosted in a toplevel [rel]atime mount. man 2 mount
has 'An attempt was made to modify (MS_REMOUNT) the MS_RDONLY, MS_NOSUID, or
MS_NOEXEC flag, or one of the "atime" flags (MS_NOATIME, MS_NODIRATIME, MS_RELATIME)
of an existing mount, but the mount is locked'.

b) lxc has default apparmor rules of
https://github.com/lxc/lxc/blob/main/config/apparmor/abstractions/container-base
where the closest match is:  "mount options=(ro,remount,bind,nodev,nosuid)"
so additional 'MS_SILENT' or 'MS_REC' flags similarly also cause the remount to
be denied

So if we use a more recognized set of options we work out of the box in
the default lxc configuration.

Signed-off-by: Caolán McNamara <caolan.mcnamara@collabora.com>
Change-Id: I5f8de2de998ae1a85fefc1c9537b79b2b3bdefec
This commit is contained in:
Caolán McNamara 2024-07-16 16:09:43 +01:00 committed by Miklos Vajna
parent 9719250e73
commit 5fbc4bbbdd
3 changed files with 18 additions and 20 deletions
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30
common/CoolMount.cpp
View file

@ -29,13 +29,11 @@
#define MOUNT mount_wrapper
#define MS_MGC_VAL 0
#define MS_SILENT 0
#define MS_NODEV 0
#define MS_UNBINDABLE 0
#define MS_BIND 1
#define MS_REC 2
#define MS_REMOUNT 4
#define MS_NOATIME 8
#define MS_NOSUID 16
#define MS_RDONLY 32
@ -82,9 +80,6 @@ int mount_wrapper(const char *source, const char *target,
if(mountflags & MS_REMOUNT)
freebsd_flags |= MNT_UPDATE;
if(mountflags & MS_NOATIME)
freebsd_flags |= MNT_NOATIME;
if(mountflags & MS_NOSUID)
freebsd_flags |= MNT_NOSUID;
@ -147,7 +142,7 @@ void usage(const char* program)
fprintf(stderr, " -u to unmount the target.\n");
}
int domount(bool namespace_mount, int argc, const char* const* argv)
int domount(int argc, const char* const* argv)
{
const char* program = argv[0];
if (argc < 3)
@ -300,16 +295,19 @@ int domount(bool namespace_mount, int argc, const char* const* argv)
else if (strcmp(option, "-r") == 0) // Readonly Mount.
{
// Now we need to set read-only and other flags with a remount.
unsigned long mountflags = (MS_BIND | MS_REC | MS_REMOUNT | MS_NODEV | MS_NOSUID
| MS_RDONLY | MS_SILENT);
// In the linux namespace mount case, setting MS_NOATIME results in EPERM on remounting
// something hosted in a toplevel [rel]atime mount. man 2 mount states 'An attempt was
// made to modify (MS_REMOUNT) the MS_RDONLY, MS_NOSUID, or MS_NOEXEC flag, or one of
// the "atime" flags (MS_NOATIME, MS_NODIRATIME, MS_RELATIME) of an existing mount, but
// the mount is locked'. Presumably we can add flags that drop privs, but not those
// that could circumvent original mount policy.
if (!namespace_mount)
mountflags |= MS_NOATIME;
unsigned long mountflags = (MS_BIND | MS_REMOUNT | MS_NODEV | MS_NOSUID | MS_RDONLY);
/* a) In the linux namespace mount case an additional MS_NOATIME, etc. will result in
EPERM on remounting something hosted in a toplevel [rel]atime mount. man 2 mount
has 'An attempt was made to modify (MS_REMOUNT) the MS_RDONLY, MS_NOSUID, or
MS_NOEXEC flag, or one of the "atime" flags (MS_NOATIME, MS_NODIRATIME, MS_RELATIME)
of an existing mount, but the mount is locked'.
b) lxc has default apparmor rules of
https://github.com/lxc/lxc/blob/main/config/apparmor/abstractions/container-base
where the closest match is: "mount options=(ro,remount,bind,nodev,nosuid)"
so additional 'MS_SILENT' or 'MS_REC' flags cause the remount to be denied
*/
int retval = MOUNT(source, target, nullptr, mountflags, nullptr);
if (retval)
{

4
common/JailUtil.cpp
View file

@ -33,7 +33,7 @@
#include "Log.hpp"
#include <SigUtil.hpp>
extern int domount(bool namespace_mount, int argc, const char* const* argv);
extern int domount(int argc, const char* const* argv);
namespace JailUtil
{
@ -130,7 +130,7 @@ bool coolmount(const std::string& arg, std::string source, std::string target)
argv[argc++] = source.c_str();
if (!target.empty())
argv[argc++] = target.c_str();
return domount(true, argc, argv) == EX_OK;
return domount(argc, argv) == EX_OK;
}
const std::string cmd = Poco::Path(Util::getApplicationPath(), "coolmount").toString() + ' '

4
tools/mount.cpp
View file

@ -19,7 +19,7 @@
#include <security.h>
extern int domount(bool namespace_mount, int argc, const char* const* argv);
extern int domount(int argc, const char* const* argv);
int main(int argc, char** argv)
{
@ -31,7 +31,7 @@ int main(int argc, char** argv)
/*WARNING*/ }
/*WARNING: PRIVILEGED CODE CHECKING END */
return domount(false, argc, argv);
return domount(argc, argv);
}
/* vim:set shiftwidth=4 softtabstop=4 expandtab: */