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SubjectRe: [PATCH 3/3] init: use do_mount() instead of ksys_mount()
On Mon, Dec 16, 2019 at 02:35:36AM +0100, megi xff wrote:
> Hello,
>
> On Thu, Dec 12, 2019 at 02:57:24PM +0100, Dominik Brodowski wrote:
> > In prepare_namespace(), do_mount() can be used instead of ksys_mount()
> > as the first and third argument are const strings in the kernel, the
> > second and fourth argument are passed through anyway, and the fifth
> > argument is NULL.
> >
> > In do_mount_root(), ksys_mount() is called with the first and third
> > argument being already kernelspace strings, which do not need to be
> > copied over from userspace to kernelspace (again). The second and
> > fourth arguments are passed through to do_mount() anyway. The fifth
> > argument, while already residing in kernelspace, needs to be put into
> > a page of its own. Then, do_mount() can be used instead of
> > ksys_mount().
> >
> > Once this is done, there are no in-kernel users to ksys_mount() left,
> > which can therefore be removed.
> >
> > Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>
> > ---
> > fs/namespace.c | 10 ++--------
> > include/linux/syscalls.h | 2 --
> > init/do_mounts.c | 28 ++++++++++++++++++++++------
> > 3 files changed, 24 insertions(+), 16 deletions(-)
> >
> > diff --git a/fs/namespace.c b/fs/namespace.c
> > index 2fd0c8bcb8c1..be601d3a8008 100644
> > --- a/fs/namespace.c
> > +++ b/fs/namespace.c
> > @@ -3325,8 +3325,8 @@ struct dentry *mount_subtree(struct vfsmount *m, const char *name)
> > }
> > EXPORT_SYMBOL(mount_subtree);
> >
> > -int ksys_mount(const char __user *dev_name, const char __user *dir_name,
> > - const char __user *type, unsigned long flags, void __user *data)
> > +SYSCALL_DEFINE5(mount, char __user *, dev_name, char __user *, dir_name,
> > + char __user *, type, unsigned long, flags, void __user *, data)
> > {
> > int ret;
> > char *kernel_type;
> > @@ -3359,12 +3359,6 @@ int ksys_mount(const char __user *dev_name, const char __user *dir_name,
> > return ret;
> > }
> >
> > -SYSCALL_DEFINE5(mount, char __user *, dev_name, char __user *, dir_name,
> > - char __user *, type, unsigned long, flags, void __user *, data)
> > -{
> > - return ksys_mount(dev_name, dir_name, type, flags, data);
> > -}
> > -
> > /*
> > * Create a kernel mount representation for a new, prepared superblock
> > * (specified by fs_fd) and attach to an open_tree-like file descriptor.
> > diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
> > index d0391cc2dae9..5262b7a76d39 100644
> > --- a/include/linux/syscalls.h
> > +++ b/include/linux/syscalls.h
> > @@ -1231,8 +1231,6 @@ asmlinkage long sys_ni_syscall(void);
> > * the ksys_xyzyyz() functions prototyped below.
> > */
> >
> > -int ksys_mount(const char __user *dev_name, const char __user *dir_name,
> > - const char __user *type, unsigned long flags, void __user *data);
> > int ksys_umount(char __user *name, int flags);
> > int ksys_dup(unsigned int fildes);
> > int ksys_chroot(const char __user *filename);
> > diff --git a/init/do_mounts.c b/init/do_mounts.c
> > index 43f6d098c880..f55cbd9cb818 100644
> > --- a/init/do_mounts.c
> > +++ b/init/do_mounts.c
> > @@ -387,12 +387,25 @@ static void __init get_fs_names(char *page)
> > *s = '\0';
> > }
> >
> > -static int __init do_mount_root(char *name, char *fs, int flags, void *data)
> > +static int __init do_mount_root(const char *name, const char *fs,
> > + const int flags, const void *data)
> > {
> > struct super_block *s;
> > - int err = ksys_mount(name, "/root", fs, flags, data);
> > - if (err)
> > - return err;
> > + char *data_page;
> > + struct page *p;
> > + int ret;
> > +
> > + /* do_mount() requires a full page as fifth argument */
> > + p = alloc_page(GFP_KERNEL);
> > + if (!p)
> > + return -ENOMEM;
> > +
> > + data_page = page_address(p);
> > + strncpy(data_page, data, PAGE_SIZE - 1);
>
> I tried 5.2-rc2 and I get kernel OOPS/panic here (do_mount_root gets inlined
> into mount_block_root):

5.5-rc2 of course. :)

> Unable to handle kernel NULL pointer dereference at virtual address 0000000000000000
> Mem abort info:
> ESR = 0x96000005
> EC = 0x25: DABT (current EL), IL = 32 bits
> SET = 0, FnV = 0
> EA = 0, S1PTW = 0
> Data abort info:
> ISV = 0, ISS = 0x00000005
> CM = 0, WnR = 0
> [0000000000000000] user address but active_mm is swapper
> Internal error: Oops: 96000005 [#1] SMP
> Modules linked in:
> CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.5.0-rc2-00128-ge93c94cc04ae #5
> Hardware name: OrangePi 3 (DT)
> pstate: 80000005 (Nzcv daif -PAN -UAO)
> pc : strncpy+0x10/0x30
> lr : do_mount_root+0x70/0x114
> sp : ffffffc01002bd20
> x29: ffffffc01002bd20 x28: 0000000000000000
> x27: 0000000000000000 x26: ffffffc011400468
> x25: ffffffc0110d23a0 x24: ffffffff01c396c0
> x23: 0000000000008000 x22: ffffff8078e5b000
> x21: ffffffc0110d23a0 x20: ffffffc0110d2220
> x19: ffffffff01c39700 x18: 00000000fffffffe
> x17: 00000000dbaba8be x16: 0000000000000000
> x15: ffffffffffffffff x14: ffffff0000000000
> x13: ffffff807b490000 x12: 0000000000000000
> x11: 0000000000000000 x10: ffffff807bb96120
> x9 : 0000000000000000 x8 : 0000000000000000
> x7 : ffffffc06a6e6000 x6 : 0000000000000000
> x5 : 000000000003b820 x4 : ffffff8078e5cfff
> x3 : 0000000000000201 x2 : ffffff8078e5c000
> x1 : 0000000000000000 x0 : ffffff8078e5c000
> Call trace:
> strncpy+0x10/0x30
> mount_block_root+0x100/0x224
> mount_root+0x10c/0x124
> prepare_namespace+0x12c/0x168
> kernel_init_freeable+0x214/0x258
> kernel_init+0x10/0xfc
> ret_from_fork+0x10/0x1c
> Code: b4000142 8b020004 aa0003e2 d503201f (39400023)
> ---[ end trace b72d58d1ea940426 ]---
> Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000000b
> SMP: stopping secondary CPUs
> Kernel Offset: disabled
> CPU features: 0x00002,20002000
> Memory Limit: none
> Rebooting in 3 seconds..
>
> regards,
> o.
>
> > + ret = do_mount(name, "/root", fs, flags, data_page);
> > + if (ret)
> > + goto out;
> >
> > ksys_chdir("/root");
> > s = current->fs->pwd.dentry->d_sb;
> > @@ -402,7 +415,10 @@ static int __init do_mount_root(char *name, char *fs, int flags, void *data)
> > s->s_type->name,
> > sb_rdonly(s) ? " readonly" : "",
> > MAJOR(ROOT_DEV), MINOR(ROOT_DEV));
> > - return 0;
> > +
> > +out:
> > + put_page(p);
> > + return ret;
> > }
> >
> > void __init mount_block_root(char *name, int flags)
> > @@ -671,7 +687,7 @@ void __init prepare_namespace(void)
> > mount_root();
> > out:
> > devtmpfs_mount();
> > - ksys_mount(".", "/", NULL, MS_MOVE, NULL);
> > + do_mount(".", "/", NULL, MS_MOVE, NULL);
> > ksys_chroot(".");
> > }
> >
> > --
> > 2.24.1
> >

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