On 5/28/24 4:54 PM, benja...@sipsolutions.net wrote: > From: Benjamin Berg <benjamin.b...@intel.com> > > Newer glibc versions are enabling rseq support by default. This remains > enabled in the cloned child process, potentially causing the host kernel > to write/read memory in the child. > > It appears that this was purely not an issue because the used memory > area happened to be above TASK_SIZE and remains mapped.
I also encountered this issue. In my case, with "Force a static link" (CONFIG_STATIC_LINK) enabled, UML will crash immediately every time it starts up. I worked around this by setting the glibc.pthread.rseq tunable via GLIBC_TUNABLES [1] before launching UML. So another easy way to work around this issue without introducing runtime overhead might be to add the GLIBC_TUNABLES=glibc.pthread.rseq=0 environment variable and exec /proc/self/exe in UML on startup. [1] https://www.gnu.org/software/libc/manual/html_node/Tunables.html Regards, Tiwei > > Note that a better approach would be to exec a small static binary that > does not link with other libraries. Using a memfd and execveat the > binary could be embedded into UML itself and it would result in an > entirely clean execution environment for userspace. > > Signed-off-by: Benjamin Berg <benjamin.b...@intel.com> > --- > arch/um/os-Linux/skas/process.c | 54 ++++++++++++++++++++++++++++++--- > 1 file changed, 50 insertions(+), 4 deletions(-) > > diff --git a/arch/um/os-Linux/skas/process.c b/arch/um/os-Linux/skas/process.c > index 41a288dcfc34..ee332a2aeea6 100644 > --- a/arch/um/os-Linux/skas/process.c > +++ b/arch/um/os-Linux/skas/process.c > @@ -255,6 +255,31 @@ static int userspace_tramp(void *stack) > int userspace_pid[NR_CPUS]; > int kill_userspace_mm[NR_CPUS]; > > +struct tramp_data { > + int pid; > + void *clone_sp; > + void *stack; > +}; > + > +static int userspace_tramp_clone_vm(void *data) > +{ > + struct tramp_data *tramp_data = data; > + > + /* > + * This helper exist to do a double-clone. First with CLONE_VM which > + * effectively disables things like rseq, and then the second one to > + * get a new memory space. > + */ > + > + tramp_data->pid = clone(userspace_tramp, tramp_data->clone_sp, > + CLONE_PARENT | CLONE_FILES | SIGCHLD, > + tramp_data->stack); > + if (tramp_data->pid < 0) > + tramp_data->pid = -errno; > + > + exit(0); > +} > + > /** > * start_userspace() - prepare a new userspace process > * @stub_stack: pointer to the stub stack. > @@ -268,9 +293,10 @@ int kill_userspace_mm[NR_CPUS]; > */ > int start_userspace(unsigned long stub_stack) > { > + struct tramp_data tramp_data; > void *stack; > unsigned long sp; > - int pid, status, n, flags, err; > + int pid, status, n, err; > > /* setup a temporary stack page */ > stack = mmap(NULL, UM_KERN_PAGE_SIZE, > @@ -286,10 +312,13 @@ int start_userspace(unsigned long stub_stack) > /* set stack pointer to the end of the stack page, so it can grow > downwards */ > sp = (unsigned long)stack + UM_KERN_PAGE_SIZE; > > - flags = CLONE_FILES | SIGCHLD; > + tramp_data.stack = (void *) stub_stack; > + tramp_data.clone_sp = (void *) sp; > + tramp_data.pid = -EINVAL; > > /* clone into new userspace process */ > - pid = clone(userspace_tramp, (void *) sp, flags, (void *) stub_stack); > + pid = clone(userspace_tramp_clone_vm, (void *) sp, > + CLONE_VM | CLONE_FILES | SIGCHLD, &tramp_data); > if (pid < 0) { > err = -errno; > printk(UM_KERN_ERR "%s : clone failed, errno = %d\n", > @@ -305,7 +334,24 @@ int start_userspace(unsigned long stub_stack) > __func__, errno); > goto out_kill; > } > - } while (WIFSTOPPED(status) && (WSTOPSIG(status) == SIGALRM)); > + } while (!WIFEXITED(status)); > + > + pid = tramp_data.pid; > + if (pid < 0) { > + printk(UM_KERN_ERR "%s : second clone failed, errno = %d\n", > + __func__, -pid); > + return pid; > + } > + > + do { > + CATCH_EINTR(n = waitpid(pid, &status, WUNTRACED | __WALL)); > + if (n < 0) { > + err = -errno; > + printk(UM_KERN_ERR "%s : wait failed, errno = %d\n", > + __func__, errno); > + goto out_kill; > + } > + } while (WIFEXITED(status) && (WSTOPSIG(status) == SIGALRM)); > > if (!WIFSTOPPED(status) || (WSTOPSIG(status) != SIGSTOP)) { > err = -EINVAL;
