On Fri, Nov 21, 2025 at 11:07:48AM -0500, Peter Xu wrote: > On Thu, Nov 20, 2025 at 01:57:33PM -0500, Ben Chaney wrote: > > Fix an issue where cpr transfer fails when running with the > > -run-with user=$USERID with a permission denied error. This issue > > occurs because the destination host creates the transfer sockets before > > it drops permissions while the source host tries to connect after > > dropping permissions. Fix this by changing the ownership of the cpr > > socket to the lower level so it is accessible to both parties. > > > > Resolves: > > https://lore.kernel.org/all/[email protected]/ > > Signed-off-by: Ben Chaney <[email protected]> > > --- > > include/system/os-posix.h | 1 + > > include/system/os-wasm.h | 1 + > > include/system/os-win32.h | 1 + > > os-posix.c | 12 ++++++++++++ > > stubs/meson.build | 1 + > > stubs/os-file.c | 6 ++++++ > > util/qemu-sockets.c | 6 ++++++ > > 7 files changed, 28 insertions(+) > > create mode 100644 stubs/os-file.c > > > > diff --git a/include/system/os-posix.h b/include/system/os-posix.h > > index ce5b3bccf8..e4b69fc316 100644 > > --- a/include/system/os-posix.h > > +++ b/include/system/os-posix.h > > @@ -54,6 +54,7 @@ void os_set_chroot(const char *path); > > void os_setup_limits(void); > > void os_setup_post(void); > > int os_mlock(bool on_fault); > > +void os_set_socket_permissions(const char *path); > > > > /** > > * qemu_alloc_stack: > > diff --git a/include/system/os-wasm.h b/include/system/os-wasm.h > > index 3abb3aaa03..eeac092183 100644 > > --- a/include/system/os-wasm.h > > +++ b/include/system/os-wasm.h > > @@ -57,6 +57,7 @@ static inline int os_set_daemonize(bool d) > > }; > > bool is_daemonized(void); > > static inline void os_daemonize(void) {} > > +static inline void os_set_socket_permissions(const char *dummy) {}; > > > > /** > > * qemu_alloc_stack: > > diff --git a/include/system/os-win32.h b/include/system/os-win32.h > > index 22d72babdf..79e42ec297 100644 > > --- a/include/system/os-win32.h > > +++ b/include/system/os-win32.h > > @@ -103,6 +103,7 @@ static inline void os_setup_post(void) {} > > static inline void os_set_proc_name(const char *dummy) {} > > void os_set_line_buffering(void); > > void os_setup_early_signal_handling(void); > > +static inline void os_set_socket_permissions(const char *dummy) {}; > > > > int getpagesize(void); > > > > diff --git a/os-posix.c b/os-posix.c > > index 52925c23d3..bbd17ff2b9 100644 > > --- a/os-posix.c > > +++ b/os-posix.c > > @@ -94,6 +94,18 @@ static struct passwd *user_pwd; /* NULL non-NULL > > NULL */ > > static uid_t user_uid = (uid_t)-1; /* -1 -1 >=0 */ > > static gid_t user_gid = (gid_t)-1; /* -1 -1 >=0 */ > > > > +void os_set_socket_permissions(const char *path) > > +{ > > + uid_t uid = user_pwd ? user_pwd->pw_uid : user_uid; > > + gid_t gid = user_pwd ? user_pwd->pw_gid : user_gid; > > + > > + if (chown(path, uid, gid) < 0) { > > + error_report("Failed to chown socket %s: %s", path, > > strerror(errno)); > > + exit(1); > > + } > > +} > > + > > + > > /* > > * Prepare to change user ID. user_id can be one of 3 forms: > > * - a username, in which case user ID will be changed to its uid, > > diff --git a/stubs/meson.build b/stubs/meson.build > > index 0b2778c568..4a4342344b 100644 > > --- a/stubs/meson.build > > +++ b/stubs/meson.build > > @@ -24,6 +24,7 @@ if have_block > > stub_ss.add(files('ram-block.c')) > > stub_ss.add(files('runstate-check.c')) > > stub_ss.add(files('uuid.c')) > > + stub_ss.add(files('os-file.c')) > > endif > > > > if have_block or have_ga > > diff --git a/stubs/os-file.c b/stubs/os-file.c > > new file mode 100644 > > index 0000000000..c32cbc7efa > > --- /dev/null > > +++ b/stubs/os-file.c > > @@ -0,0 +1,6 @@ > > +/* SPDX-License-Identifier: GPL-2.0-or-later */ > > +#include "qemu/osdep.h" > > + > > +void os_set_socket_permissions(const char *dummy) > > +{ > > +} > > diff --git a/util/qemu-sockets.c b/util/qemu-sockets.c > > index 4773755fd5..77b2d9287b 100644 > > --- a/util/qemu-sockets.c > > +++ b/util/qemu-sockets.c > > @@ -1026,6 +1026,12 @@ static int unix_listen_saddr(UnixSocketAddress > > *saddr, > > error_setg_errno(errp, errno, "Failed to bind socket to %s", path); > > goto err; > > } > > + /* > > + * Change the permissions on the socket to match the permission of the > > + * counterparty process > > + */ > > + os_set_socket_permissions(un.sun_path); > > QEMU's -run-with described user= as: > > user=username or user=uid:gid can be used to drop root privileges before > starting guest execution. QEMU will use the setuid and setgid system > calls to switch to the specified identity... > > So it explicitly mentioned "before starting guest execution", hence at > least from that doc it's hard to say if unix socket should be created with > the privilege or after dropped.. Here I believe cpr socket should be the > case for latter, however when it's a generic change to unix listening > ports, I wonder if there's other unwanted side effects. > > Considering unix socket itself doesn't really have a UID attached to it, > it's only the unix path that needs a chmod(), meanwhile the mgmt of course > knows both the right UID (as specified in -run-with) and the path, would it > make sense if the mgmt chmod() after it starts dest QEMU? That'll reduce > the scope of impact to minimum.
My guiding principal is that the behaviour of cold-plugging a backend on the CLI and hot-plugging a backend via QMP should be the same, since we want to eventually get to a point where everything can be configured via QMP and zero CLI except for basic process related things. hot-plugging will honour -run-with setting, therefore cold-plugging should do the same. Having said that, the run-with setting impl is very clearcut however that it is intended to run *after* all cold-plug setup is complete. IOW, it is inherantly intending to have differnt behaviour for cold plug vs hotplug. There's a very real risk we'll cause regressions if we change how permissions are handled implicitly on any backends. With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
