| Message ID | 20200804203155.2181099-1-lokeshgidra@google.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | Userfaultfd: Avoid double free of userfault_ctx and remove O_CLOEXEC | expand |
On Tue, Aug 04, 2020 at 01:31:55PM -0700, Lokesh Gidra wrote: > when get_unused_fd_flags returns error, ctx will be freed by > userfaultfd's release function, which is indirectly called by fput(). > Also, if anon_inode_getfile_secure() returns an error, then > userfaultfd_ctx_put() is called, which calls mmdrop() and frees ctx. > > Also, the O_CLOEXEC was inadvertently added to the call to > get_unused_fd_flags() [1]. > > Adding Al Viro's suggested-by, based on [2]. > > [1] https://lore.kernel.org/lkml/1f69c0ab-5791-974f-8bc0-3997ab1d61ea@dancol.org/ > [2] https://lore.kernel.org/lkml/20200719165746.GJ2786714@ZenIV.linux.org.uk/ > > Fixes: d08ac70b1e0d (Wire UFFD up to SELinux) > Suggested-by: Al Viro <viro@zeniv.linux.org.uk> > Reported-by: syzbot+75867c44841cb6373570@syzkaller.appspotmail.com > Signed-off-by: Lokesh Gidra <lokeshgidra@google.com> What branch does this patch apply to? Neither mainline nor linux-next works. - Eric
On Tue, Aug 4, 2020 at 1:45 PM Eric Biggers <ebiggers@kernel.org> wrote: > > On Tue, Aug 04, 2020 at 01:31:55PM -0700, Lokesh Gidra wrote: > > when get_unused_fd_flags returns error, ctx will be freed by > > userfaultfd's release function, which is indirectly called by fput(). > > Also, if anon_inode_getfile_secure() returns an error, then > > userfaultfd_ctx_put() is called, which calls mmdrop() and frees ctx. > > > > Also, the O_CLOEXEC was inadvertently added to the call to > > get_unused_fd_flags() [1]. > > > > Adding Al Viro's suggested-by, based on [2]. > > > > [1] https://lore.kernel.org/lkml/1f69c0ab-5791-974f-8bc0-3997ab1d61ea@dancol.org/ > > [2] https://lore.kernel.org/lkml/20200719165746.GJ2786714@ZenIV.linux.org.uk/ > > > > Fixes: d08ac70b1e0d (Wire UFFD up to SELinux) > > Suggested-by: Al Viro <viro@zeniv.linux.org.uk> > > Reported-by: syzbot+75867c44841cb6373570@syzkaller.appspotmail.com > > Signed-off-by: Lokesh Gidra <lokeshgidra@google.com> > > What branch does this patch apply to? Neither mainline nor linux-next works. > On James Morris' tree (secure_uffd_v5.9 branch). > - Eric
On Tue, Aug 04, 2020 at 01:49:30PM -0700, Lokesh Gidra wrote: > On Tue, Aug 4, 2020 at 1:45 PM Eric Biggers <ebiggers@kernel.org> wrote: > > > > On Tue, Aug 04, 2020 at 01:31:55PM -0700, Lokesh Gidra wrote: > > > when get_unused_fd_flags returns error, ctx will be freed by > > > userfaultfd's release function, which is indirectly called by fput(). > > > Also, if anon_inode_getfile_secure() returns an error, then > > > userfaultfd_ctx_put() is called, which calls mmdrop() and frees ctx. > > > > > > Also, the O_CLOEXEC was inadvertently added to the call to > > > get_unused_fd_flags() [1]. > > > > > > Adding Al Viro's suggested-by, based on [2]. > > > > > > [1] https://lore.kernel.org/lkml/1f69c0ab-5791-974f-8bc0-3997ab1d61ea@dancol.org/ > > > [2] https://lore.kernel.org/lkml/20200719165746.GJ2786714@ZenIV.linux.org.uk/ > > > > > > Fixes: d08ac70b1e0d (Wire UFFD up to SELinux) > > > Suggested-by: Al Viro <viro@zeniv.linux.org.uk> > > > Reported-by: syzbot+75867c44841cb6373570@syzkaller.appspotmail.com > > > Signed-off-by: Lokesh Gidra <lokeshgidra@google.com> > > > > What branch does this patch apply to? Neither mainline nor linux-next works. > > > On James Morris' tree (secure_uffd_v5.9 branch). > For those of us not "in the know", that apparently means branch secure_uffd_v5.9 of https://git.kernel.org/pub/scm/linux/kernel/git/jmorris/linux-security.git Perhaps it would make more sense to resend your original patch series with this fix folded in? > diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c > index ae859161908f..e15eb8fdc083 100644 > --- a/fs/userfaultfd.c > +++ b/fs/userfaultfd.c > @@ -2042,24 +2042,18 @@ SYSCALL_DEFINE1(userfaultfd, int, flags) > O_RDWR | (flags & UFFD_SHARED_FCNTL_FLAGS), > NULL); > if (IS_ERR(file)) { > - fd = PTR_ERR(file); > - goto out; > + userfaultfd_ctx_put(ctx); > + return PTR_ERR(file); > } > > - fd = get_unused_fd_flags(O_RDONLY | O_CLOEXEC); > + fd = get_unused_fd_flags(O_RDONLY); > if (fd < 0) { > fput(file); > - goto out; > + return fd; > } > > ctx->owner = file_inode(file); > fd_install(fd, file); > - > -out: > - if (fd < 0) { > - mmdrop(ctx->mm); > - kmem_cache_free(userfaultfd_ctx_cachep, ctx); > - } > return fd; This introduces the opposite bug: now it's hardcoded to *not* use O_CLOEXEC, instead of using the flag the user passed in the flags argument to the syscall. - Eric
On Tue, Aug 4, 2020 at 1:58 PM Eric Biggers <ebiggers@kernel.org> wrote: > > On Tue, Aug 04, 2020 at 01:49:30PM -0700, Lokesh Gidra wrote: > > On Tue, Aug 4, 2020 at 1:45 PM Eric Biggers <ebiggers@kernel.org> wrote: > > > > > > On Tue, Aug 04, 2020 at 01:31:55PM -0700, Lokesh Gidra wrote: > > > > when get_unused_fd_flags returns error, ctx will be freed by > > > > userfaultfd's release function, which is indirectly called by fput(). > > > > Also, if anon_inode_getfile_secure() returns an error, then > > > > userfaultfd_ctx_put() is called, which calls mmdrop() and frees ctx. > > > > > > > > Also, the O_CLOEXEC was inadvertently added to the call to > > > > get_unused_fd_flags() [1]. > > > > > > > > Adding Al Viro's suggested-by, based on [2]. > > > > > > > > [1] https://lore.kernel.org/lkml/1f69c0ab-5791-974f-8bc0-3997ab1d61ea@dancol.org/ > > > > [2] https://lore.kernel.org/lkml/20200719165746.GJ2786714@ZenIV.linux.org.uk/ > > > > > > > > Fixes: d08ac70b1e0d (Wire UFFD up to SELinux) > > > > Suggested-by: Al Viro <viro@zeniv.linux.org.uk> > > > > Reported-by: syzbot+75867c44841cb6373570@syzkaller.appspotmail.com > > > > Signed-off-by: Lokesh Gidra <lokeshgidra@google.com> > > > > > > What branch does this patch apply to? Neither mainline nor linux-next works. > > > > > On James Morris' tree (secure_uffd_v5.9 branch). > > > > For those of us not "in the know", that apparently means branch secure_uffd_v5.9 > of https://git.kernel.org/pub/scm/linux/kernel/git/jmorris/linux-security.git > > Perhaps it would make more sense to resend your original patch series with this > fix folded in? > OK. I'll resend the whole patch series with the fixes soon. > > diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c > > index ae859161908f..e15eb8fdc083 100644 > > --- a/fs/userfaultfd.c > > +++ b/fs/userfaultfd.c > > @@ -2042,24 +2042,18 @@ SYSCALL_DEFINE1(userfaultfd, int, flags) > > O_RDWR | (flags & UFFD_SHARED_FCNTL_FLAGS), > > NULL); > > if (IS_ERR(file)) { > > - fd = PTR_ERR(file); > > - goto out; > > + userfaultfd_ctx_put(ctx); > > + return PTR_ERR(file); > > } > > > > - fd = get_unused_fd_flags(O_RDONLY | O_CLOEXEC); > > + fd = get_unused_fd_flags(O_RDONLY); > > if (fd < 0) { > > fput(file); > > - goto out; > > + return fd; > > } > > > > ctx->owner = file_inode(file); > > fd_install(fd, file); > > - > > -out: > > - if (fd < 0) { > > - mmdrop(ctx->mm); > > - kmem_cache_free(userfaultfd_ctx_cachep, ctx); > > - } > > return fd; > > This introduces the opposite bug: now it's hardcoded to *not* use O_CLOEXEC, > instead of using the flag the user passed in the flags argument to the syscall. I get your point. I agree the flags passed in to the syscall should be used. > > - Eric
On 2020-08-04, Lokesh Gidra <lokeshgidra@google.com> wrote: > when get_unused_fd_flags returns error, ctx will be freed by > userfaultfd's release function, which is indirectly called by fput(). > Also, if anon_inode_getfile_secure() returns an error, then > userfaultfd_ctx_put() is called, which calls mmdrop() and frees ctx. > > Also, the O_CLOEXEC was inadvertently added to the call to > get_unused_fd_flags() [1]. I disagree that it is "wrong" to do O_CLOEXEC-by-default (after all, it's trivial to disable O_CLOEXEC, but it's non-trivial to enable it on an existing file descriptor because it's possible for another thread to exec() before you set the flag). Several new syscalls and fd-returning facilities are O_CLOEXEC-by-default now (the most obvious being pidfds and seccomp notifier fds). At the very least there should be a new flag added that sets O_CLOEXEC. > Adding Al Viro's suggested-by, based on [2]. > > [1] https://lore.kernel.org/lkml/1f69c0ab-5791-974f-8bc0-3997ab1d61ea@dancol.org/ > [2] https://lore.kernel.org/lkml/20200719165746.GJ2786714@ZenIV.linux.org.uk/ > > Fixes: d08ac70b1e0d (Wire UFFD up to SELinux) > Suggested-by: Al Viro <viro@zeniv.linux.org.uk> > Reported-by: syzbot+75867c44841cb6373570@syzkaller.appspotmail.com > Signed-off-by: Lokesh Gidra <lokeshgidra@google.com> > --- > fs/userfaultfd.c | 14 ++++---------- > 1 file changed, 4 insertions(+), 10 deletions(-) > > diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c > index ae859161908f..e15eb8fdc083 100644 > --- a/fs/userfaultfd.c > +++ b/fs/userfaultfd.c > @@ -2042,24 +2042,18 @@ SYSCALL_DEFINE1(userfaultfd, int, flags) > O_RDWR | (flags & UFFD_SHARED_FCNTL_FLAGS), > NULL); > if (IS_ERR(file)) { > - fd = PTR_ERR(file); > - goto out; > + userfaultfd_ctx_put(ctx); > + return PTR_ERR(file); > } > > - fd = get_unused_fd_flags(O_RDONLY | O_CLOEXEC); > + fd = get_unused_fd_flags(O_RDONLY); > if (fd < 0) { > fput(file); > - goto out; > + return fd; > } > > ctx->owner = file_inode(file); > fd_install(fd, file); > - > -out: > - if (fd < 0) { > - mmdrop(ctx->mm); > - kmem_cache_free(userfaultfd_ctx_cachep, ctx); > - } > return fd; > } > > -- > 2.28.0.163.g6104cc2f0b6-goog >
On Wed, Aug 05, 2020 at 01:47:58PM +1000, Aleksa Sarai wrote: > On 2020-08-04, Lokesh Gidra <lokeshgidra@google.com> wrote: > > when get_unused_fd_flags returns error, ctx will be freed by > > userfaultfd's release function, which is indirectly called by fput(). > > Also, if anon_inode_getfile_secure() returns an error, then > > userfaultfd_ctx_put() is called, which calls mmdrop() and frees ctx. > > > > Also, the O_CLOEXEC was inadvertently added to the call to > > get_unused_fd_flags() [1]. > > I disagree that it is "wrong" to do O_CLOEXEC-by-default (after all, > it's trivial to disable O_CLOEXEC, but it's non-trivial to enable it on > an existing file descriptor because it's possible for another thread to > exec() before you set the flag). Several new syscalls and fd-returning > facilities are O_CLOEXEC-by-default now (the most obvious being pidfds > and seccomp notifier fds). Sure, O_CLOEXEC *should* be the default, but this is an existing syscall so it has to keep the existing behavior. > At the very least there should be a new flag added that sets O_CLOEXEC. There already is one (but these patches broke it). - Eric
On Tue, Aug 4, 2020 at 9:08 PM Eric Biggers <ebiggers@kernel.org> wrote: > > On Wed, Aug 05, 2020 at 01:47:58PM +1000, Aleksa Sarai wrote: > > On 2020-08-04, Lokesh Gidra <lokeshgidra@google.com> wrote: > > > when get_unused_fd_flags returns error, ctx will be freed by > > > userfaultfd's release function, which is indirectly called by fput(). > > > Also, if anon_inode_getfile_secure() returns an error, then > > > userfaultfd_ctx_put() is called, which calls mmdrop() and frees ctx. > > > > > > Also, the O_CLOEXEC was inadvertently added to the call to > > > get_unused_fd_flags() [1]. > > > > I disagree that it is "wrong" to do O_CLOEXEC-by-default (after all, > > it's trivial to disable O_CLOEXEC, but it's non-trivial to enable it on > > an existing file descriptor because it's possible for another thread to > > exec() before you set the flag). Several new syscalls and fd-returning > > facilities are O_CLOEXEC-by-default now (the most obvious being pidfds > > and seccomp notifier fds). > > Sure, O_CLOEXEC *should* be the default, but this is an existing syscall so it > has to keep the existing behavior. > > > At the very least there should be a new flag added that sets O_CLOEXEC. > > There already is one (but these patches broke it). > I looked at the existing implementation, and the right thing is to pass on the 'flags' (that is passed in to the syscall) to fetch 'fd'. Besides, as you said in the other email thread, anon_inode_getfile_secure() should be replaced with anon_inode_getfd_secure(), which will remove this ambiguity. I'll resend the patch series soon with all the changes that you proposed. > - Eric
On 2020-08-04, Eric Biggers <ebiggers@kernel.org> wrote: > On Wed, Aug 05, 2020 at 01:47:58PM +1000, Aleksa Sarai wrote: > > On 2020-08-04, Lokesh Gidra <lokeshgidra@google.com> wrote: > > > when get_unused_fd_flags returns error, ctx will be freed by > > > userfaultfd's release function, which is indirectly called by fput(). > > > Also, if anon_inode_getfile_secure() returns an error, then > > > userfaultfd_ctx_put() is called, which calls mmdrop() and frees ctx. > > > > > > Also, the O_CLOEXEC was inadvertently added to the call to > > > get_unused_fd_flags() [1]. > > > > I disagree that it is "wrong" to do O_CLOEXEC-by-default (after all, > > it's trivial to disable O_CLOEXEC, but it's non-trivial to enable it on > > an existing file descriptor because it's possible for another thread to > > exec() before you set the flag). Several new syscalls and fd-returning > > facilities are O_CLOEXEC-by-default now (the most obvious being pidfds > > and seccomp notifier fds). > > Sure, O_CLOEXEC *should* be the default, but this is an existing syscall so it > has to keep the existing behavior. Ah, I missed that this was a UAPI breakage. :P > > At the very least there should be a new flag added that sets O_CLOEXEC. > > There already is one (but these patches broke it).
diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c index ae859161908f..e15eb8fdc083 100644 --- a/fs/userfaultfd.c +++ b/fs/userfaultfd.c @@ -2042,24 +2042,18 @@ SYSCALL_DEFINE1(userfaultfd, int, flags) O_RDWR | (flags & UFFD_SHARED_FCNTL_FLAGS), NULL); if (IS_ERR(file)) { - fd = PTR_ERR(file); - goto out; + userfaultfd_ctx_put(ctx); + return PTR_ERR(file); } - fd = get_unused_fd_flags(O_RDONLY | O_CLOEXEC); + fd = get_unused_fd_flags(O_RDONLY); if (fd < 0) { fput(file); - goto out; + return fd; } ctx->owner = file_inode(file); fd_install(fd, file); - -out: - if (fd < 0) { - mmdrop(ctx->mm); - kmem_cache_free(userfaultfd_ctx_cachep, ctx); - } return fd; }
when get_unused_fd_flags returns error, ctx will be freed by userfaultfd's release function, which is indirectly called by fput(). Also, if anon_inode_getfile_secure() returns an error, then userfaultfd_ctx_put() is called, which calls mmdrop() and frees ctx. Also, the O_CLOEXEC was inadvertently added to the call to get_unused_fd_flags() [1]. Adding Al Viro's suggested-by, based on [2]. [1] https://lore.kernel.org/lkml/1f69c0ab-5791-974f-8bc0-3997ab1d61ea@dancol.org/ [2] https://lore.kernel.org/lkml/20200719165746.GJ2786714@ZenIV.linux.org.uk/ Fixes: d08ac70b1e0d (Wire UFFD up to SELinux) Suggested-by: Al Viro <viro@zeniv.linux.org.uk> Reported-by: syzbot+75867c44841cb6373570@syzkaller.appspotmail.com Signed-off-by: Lokesh Gidra <lokeshgidra@google.com> --- fs/userfaultfd.c | 14 ++++---------- 1 file changed, 4 insertions(+), 10 deletions(-)