| Message ID | 20030808105321.GA5096@gondor.apana.org.au |
|---|---|
| State | New, archived |
| Headers | show |
| Series |
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| Related | show |
On Fri, Aug 08, 2003 at 08:53:21PM +1000, herbert wrote: > > The steal_locks() call in binfmt_elf.c is buggy. It steals locks from > a files entry whose reference was dropped much earlier. This allows it > to steal other process's locks. > > The following patch calls steal_locks() earlier so that this does not > happen. My patch is buggy too. If a file is closed by another clone between the two steal_locks calls the lock will again be lost. Fortunately this much harder to trigger than the previous bug. The following patch fixes that.
Hello, thanks a lot for analyzing this. Please see the patch I just posted unter the subject: [PATCH] 2.4.22-rc2 steal_locks and load_elf_binary cleanups That patch fixes the bug in a slightly cleaner way. On Sat, 9 Aug 2003, Herbert Xu wrote: > On Fri, Aug 08, 2003 at 08:53:21PM +1000, herbert wrote: > > > > The steal_locks() call in binfmt_elf.c is buggy. It steals locks from > > a files entry whose reference was dropped much earlier. This allows it > > to steal other process's locks. This makes sense. > > The following patch calls steal_locks() earlier so that this does not > > happen. > > My patch is buggy too. If a file is closed by another clone between > the two steal_locks calls the lock will again be lost. Fortunately > this much harder to trigger than the previous bug. > > The following patch fixes that. > - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
On Sat, 9 Aug 2003, Herbert Xu wrote: > On Fri, Aug 08, 2003 at 08:53:21PM +1000, herbert wrote: > > > > The steal_locks() call in binfmt_elf.c is buggy. It steals locks from > > a files entry whose reference was dropped much earlier. This allows it > > to steal other process's locks. > > > > The following patch calls steal_locks() earlier so that this does not > > happen. > > My patch is buggy too. If a file is closed by another clone between > the two steal_locks calls the lock will again be lost. Fortunately > this much harder to trigger than the previous bug. I think this is not a strict bug---this scenario is not covered by POSIX in the first place. Unless lock stealing is done atomically with unshare_files there is a window of oportunity between unshare_files() and steal_locks(), so locks can still get lost. Cheers, Andreas. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
On Sat, Aug 09, 2003 at 04:04:53AM +0200, Andreas Gruenbacher wrote: > > > My patch is buggy too. If a file is closed by another clone between > > the two steal_locks calls the lock will again be lost. Fortunately > > this much harder to trigger than the previous bug. > > I think this is not a strict bug---this scenario is not covered by POSIX > in the first place. Unless lock stealing is done atomically with > unshare_files there is a window of oportunity between unshare_files() and > steal_locks(), so locks can still get lost. It's not a standard compliance issue. In this case the lock will never be released and it will eventually lead to a crash when someone reads /proc/locks.
Here is an accumulate patch of Herbert's and my changes. I could see no reason for ftmp in flush_old_exec and load_elf_binary, so I removed that. Please correct me if that is wrong. Cheers, Andreas. On Fri, 8 Aug 2003, Herbert Xu wrote: > Hi: > > The steal_locks() call in binfmt_elf.c is buggy. It steals locks from > a files entry whose reference was dropped much earlier. This allows it > to steal other process's locks. > > The following patch calls steal_locks() earlier so that this does not > happen. Index: linux-2.4.22-rc2.orig/fs/exec.c =================================================================== --- linux-2.4.22-rc2.orig.orig/fs/exec.c 2003-08-09 04:44:07.000000000 +0200 +++ linux-2.4.22-rc2.orig/fs/exec.c 2003-08-09 04:47:53.000000000 +0200 @@ -582,8 +582,6 @@ int flush_old_exec(struct linux_binprm * retval = unshare_files(); if(retval) goto flush_failed; - steal_locks(files, current->files); - put_files_struct(files); /* * Release all of the old mmap stuff @@ -592,6 +590,8 @@ int flush_old_exec(struct linux_binprm * if (retval) goto mmap_failed; /* This is the point of no return */ + steal_locks(files); + put_files_struct(files); release_old_signals(oldsig); current->sas_ss_sp = current->sas_ss_size = 0; @@ -629,6 +629,8 @@ int flush_old_exec(struct linux_binprm * return 0; mmap_failed: + put_files_struct(current->files); + current->files = files; flush_failed: spin_lock_irq(¤t->sigmask_lock); if (current->sig != oldsig) { Index: linux-2.4.22-rc2.orig/include/linux/fs.h =================================================================== --- linux-2.4.22-rc2.orig.orig/include/linux/fs.h 2003-08-09 04:44:07.000000000 +0200 +++ linux-2.4.22-rc2.orig/include/linux/fs.h 2003-08-09 04:46:05.000000000 +0200 @@ -674,7 +674,7 @@ extern int __get_lease(struct inode *ino extern time_t lease_get_mtime(struct inode *); extern int lock_may_read(struct inode *, loff_t start, unsigned long count); extern int lock_may_write(struct inode *, loff_t start, unsigned long count); -extern void steal_locks(fl_owner_t from, fl_owner_t to); +extern void steal_locks(fl_owner_t from); struct fasync_struct { int magic; Index: linux-2.4.22-rc2.orig/fs/binfmt_elf.c =================================================================== --- linux-2.4.22-rc2.orig.orig/fs/binfmt_elf.c 2003-08-09 04:44:07.000000000 +0200 +++ linux-2.4.22-rc2.orig/fs/binfmt_elf.c 2003-08-09 04:48:27.000000000 +0200 @@ -444,7 +444,7 @@ static int load_elf_binary(struct linux_ struct elfhdr interp_elf_ex; struct exec interp_ex; char passed_fileno[6]; - struct files_struct *files, *ftmp; + struct files_struct *files; /* Get the exec-header */ elf_ex = *((struct elfhdr *) bprm->buf); @@ -480,7 +480,6 @@ static int load_elf_binary(struct linux_ files = current->files; /* Refcounted so ok */ if(unshare_files() < 0) goto out_free_ph; - steal_locks(files, current->files); /* exec will make our files private anyway, but for the a.out loader stuff we need to do it earlier */ @@ -603,7 +602,9 @@ static int load_elf_binary(struct linux_ goto out_free_dentry; /* Discard our unneeded old files struct */ + steal_locks(files); put_files_struct(files); + files = NULL; /* OK, This is the point of no return */ current->mm->start_data = 0; @@ -714,18 +715,16 @@ static int load_elf_binary(struct linux_ elf_entry = load_elf_interp(&interp_elf_ex, interpreter, &interp_load_addr); - - allow_write_access(interpreter); - fput(interpreter); - kfree(elf_interpreter); - if (BAD_ADDR(elf_entry)) { printk(KERN_ERR "Unable to load interpreter\n"); - kfree(elf_phdata); send_sig(SIGSEGV, current, 0); retval = -ENOEXEC; /* Nobody gets to see this, but.. */ - goto out; + goto out_free_dentry; } + + allow_write_access(interpreter); + fput(interpreter); + kfree(elf_interpreter); } kfree(elf_phdata); @@ -811,10 +810,10 @@ out_free_interp: out_free_file: sys_close(elf_exec_fileno); out_free_fh: - ftmp = current->files; - current->files = files; - steal_locks(ftmp, current->files); - put_files_struct(ftmp); + if (files) { + put_files_struct(current->files); + current->files = files; + } out_free_ph: kfree(elf_phdata); goto out; Index: linux-2.4.22-rc2.orig/fs/locks.c =================================================================== --- linux-2.4.22-rc2.orig.orig/fs/locks.c 2003-08-09 04:44:07.000000000 +0200 +++ linux-2.4.22-rc2.orig/fs/locks.c 2003-08-09 04:46:05.000000000 +0200 @@ -1937,11 +1937,11 @@ done: return length; } -void steal_locks(fl_owner_t from, fl_owner_t to) +void steal_locks(fl_owner_t from) { struct list_head *tmp; - if (from == to) + if (from == current->files) return; lock_kernel(); @@ -1949,7 +1949,7 @@ void steal_locks(fl_owner_t from, fl_own struct file_lock *fl = list_entry(tmp, struct file_lock, fl_link); if (fl->fl_owner == from) - fl->fl_owner = to; + fl->fl_owner = current->files; } unlock_kernel(); }
On Sat, Aug 09, 2003 at 04:59:03AM +0200, Andreas Gruenbacher wrote: > > @@ -714,18 +715,16 @@ static int load_elf_binary(struct linux_ > elf_entry = load_elf_interp(&interp_elf_ex, > interpreter, > &interp_load_addr); > - > - allow_write_access(interpreter); > - fput(interpreter); > - kfree(elf_interpreter); > - > if (BAD_ADDR(elf_entry)) { > printk(KERN_ERR "Unable to load interpreter\n"); > - kfree(elf_phdata); > send_sig(SIGSEGV, current, 0); > retval = -ENOEXEC; /* Nobody gets to see this, but.. */ > - goto out; > + goto out_free_dentry; > } > + > + allow_write_access(interpreter); > + fput(interpreter); > + kfree(elf_interpreter); > } This looks bad since you're past the point of no return.
On Sat, 9 Aug 2003, Herbert Xu wrote: > On Sat, Aug 09, 2003 at 04:04:53AM +0200, Andreas Gruenbacher wrote: > > > > > My patch is buggy too. If a file is closed by another clone between > > > the two steal_locks calls the lock will again be lost. Fortunately > > > this much harder to trigger than the previous bug. > > > > I think this is not a strict bug---this scenario is not covered by POSIX > > in the first place. Unless lock stealing is done atomically with > > unshare_files there is a window of oportunity between unshare_files() and > > steal_locks(), so locks can still get lost. > > It's not a standard compliance issue. In this case the lock will never > be released and it will eventually lead to a crash when someone reads > /proc/locks. I don't see how this would happen. Could you please elaborate? - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
On Sat, 9 Aug 2003, Herbert Xu wrote: > On Sat, Aug 09, 2003 at 04:59:03AM +0200, Andreas Gruenbacher wrote: > > > > @@ -714,18 +715,16 @@ static int load_elf_binary(struct linux_ > > elf_entry = load_elf_interp(&interp_elf_ex, > > interpreter, > > &interp_load_addr); > > - > > - allow_write_access(interpreter); > > - fput(interpreter); > > - kfree(elf_interpreter); > > - > > if (BAD_ADDR(elf_entry)) { > > printk(KERN_ERR "Unable to load interpreter\n"); > > - kfree(elf_phdata); > > send_sig(SIGSEGV, current, 0); > > retval = -ENOEXEC; /* Nobody gets to see this, but.. */ > > - goto out; > > + goto out_free_dentry; > > } > > + > > + allow_write_access(interpreter); > > + fput(interpreter); > > + kfree(elf_interpreter); > > } > > This looks bad since you're past the point of no return. This is an equivalence transformation except for the explicit sys_close(elf_exec_fileno) in the unwind code, which would eventually happen, anyway. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
On Sat, Aug 09, 2003 at 05:13:52AM +0200, Andreas Gruenbacher wrote: > On Sat, 9 Aug 2003, Herbert Xu wrote: > > > On Sat, Aug 09, 2003 at 04:04:53AM +0200, Andreas Gruenbacher wrote: > > > > > > > My patch is buggy too. If a file is closed by another clone between > > > > the two steal_locks calls the lock will again be lost. Fortunately > > > > this much harder to trigger than the previous bug. > > > > > > I think this is not a strict bug---this scenario is not covered by POSIX > > > in the first place. Unless lock stealing is done atomically with > > > unshare_files there is a window of oportunity between unshare_files() and > > > steal_locks(), so locks can still get lost. > > > > It's not a standard compliance issue. In this case the lock will never > > be released and it will eventually lead to a crash when someone reads > > /proc/locks. > > I don't see how this would happen. Could you please elaborate? Suppose that A and B share current->files and fd has a POSIX lock on it. A B unshare_files steal_locks close(fd) exec fails steal_locks put_files_struct The close in B fails to release the lock as it has been stolen by the new files structure. The second steal_locks sets the fl_owner back to the original files structure which no longer has fd in it and hence can never release that lock. The put_files_struct doesn't release the lock either since it is now owned by the original file structure.
On Sat, 2003-08-09 at 05:19, Herbert Xu wrote: > On Sat, Aug 09, 2003 at 05:13:52AM +0200, Andreas Gruenbacher wrote: > > On Sat, 9 Aug 2003, Herbert Xu wrote: > > > > > On Sat, Aug 09, 2003 at 04:04:53AM +0200, Andreas Gruenbacher wrote: > > > > > > > > > My patch is buggy too. If a file is closed by another clone between > > > > > the two steal_locks calls the lock will again be lost. Fortunately > > > > > this much harder to trigger than the previous bug. > > > > > > > > I think this is not a strict bug---this scenario is not covered by POSIX > > > > in the first place. Unless lock stealing is done atomically with > > > > unshare_files there is a window of oportunity between unshare_files() and > > > > steal_locks(), so locks can still get lost. > > > > > > It's not a standard compliance issue. In this case the lock will never > > > be released and it will eventually lead to a crash when someone reads > > > /proc/locks. > > > > I don't see how this would happen. Could you please elaborate? > > Suppose that A and B share current->files and fd has a POSIX lock on it. > > A B > unshare_files > steal_locks > close(fd) > exec fails > steal_locks > put_files_struct > > The close in B fails to release the lock as it has been stolen by the > new files structure. The second steal_locks sets the fl_owner back to > the original files structure which no longer has fd in it and hence can > never release that lock. The put_files_struct doesn't release the lock > either since it is now owned by the original file structure. In the patch I've sent there is no stealing back of locks, so that case does not exist. Cheers,
--- kernel-source-2.4/fs/binfmt_elf.c.orig 2003-08-08 20:46:56.000000000 +1000 +++ kernel-source-2.4/fs/binfmt_elf.c 2003-08-08 20:47:05.000000000 +1000 @@ -480,6 +480,7 @@ files = current->files; /* Refcounted so ok */ if(unshare_files() < 0) goto out_free_ph; + steal_locks(files, current->files); /* exec will make our files private anyway, but for the a.out loader stuff we need to do it earlier */ @@ -797,7 +798,6 @@ if (current->ptrace & PT_PTRACED) send_sig(SIGTRAP, current, 0); retval = 0; - steal_locks(files, current->files); out: return retval; @@ -813,6 +813,7 @@ out_free_fh: ftmp = current->files; current->files = files; + steal_locks(ftmp, current->files); put_files_struct(ftmp); out_free_ph: kfree(elf_phdata);