[PATCH] mm: Avoid creating virtual address aliases in brk()/mmap()/mremap()

[PATCH] mm: Avoid creating virtual address aliases in brk()/mmap()/mremap()

[Catalin Marinas]

Currently the arm64 kernel ignores the top address byte passed to brk(),
mmap() and mremap(). When the user is not aware of the 56-bit address
limit or relies on the kernel to return an error, untagging such
pointers has the potential to create address aliases in user-space.
Passing a tagged address to munmap(), madvise() is permitted since the
tagged pointer is expected to be inside an existing mapping.

Remove untagging in the above functions by partially reverting commit
ce18d171cb73 ("mm: untag user pointers in mmap/munmap/mremap/brk"). In
addition, update the arm64 tagged-address-abi.rst document accordingly.

Fixes: ce18d171cb73 ("mm: untag user pointers in mmap/munmap/mremap/brk")
Cc: <stable@vger.kernel.org> # 5.4.x-
Cc: Andrey Konovalov <andreyknvl@google.com>
Cc: Will Deacon <will@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Florian Weimer <fweimer@redhat.com>
Reported-by: Victor Stinner <vstinner@redhat.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
---
 Documentation/arm64/tagged-address-abi.rst | 7 +++++--
 mm/mmap.c                                  | 4 ----
 mm/mremap.c                                | 1 -
 3 files changed, 5 insertions(+), 7 deletions(-)

diff --git a/Documentation/arm64/tagged-address-abi.rst b/Documentation/arm64/tagged-address-abi.rst
index d4a85d535bf9..1771a8b5712e 100644
--- a/Documentation/arm64/tagged-address-abi.rst
+++ b/Documentation/arm64/tagged-address-abi.rst
@@ -44,8 +44,11 @@ The AArch64 Tagged Address ABI has two stages of relaxation depending
 how the user addresses are used by the kernel:
 
 1. User addresses not accessed by the kernel but used for address space
-   management (e.g. ``mmap()``, ``mprotect()``, ``madvise()``). The use
-   of valid tagged pointers in this context is always allowed.
+   management (e.g. ``mprotect()``, ``madvise()``). The use of valid
+   tagged pointers in this context is allowed with the exception of
+   ``brk()``, ``mmap()`` and the ``new_address`` argument to
+   ``mremap()`` as these have the potential of aliasing with existing
+   user addresses.
 
 2. User addresses accessed by the kernel (e.g. ``write()``). This ABI
    relaxation is disabled by default and the application thread needs to
diff --git a/mm/mmap.c b/mm/mmap.c
index 6756b8bb0033..d681a20eb4ea 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -195,8 +195,6 @@ SYSCALL_DEFINE1(brk, unsigned long, brk)
 	bool downgraded = false;
 	LIST_HEAD(uf);
 
-	brk = untagged_addr(brk);
-
 	if (down_write_killable(&mm->mmap_sem))
 		return -EINTR;
 
@@ -1557,8 +1555,6 @@ unsigned long ksys_mmap_pgoff(unsigned long addr, unsigned long len,
 	struct file *file = NULL;
 	unsigned long retval;
 
-	addr = untagged_addr(addr);
-
 	if (!(flags & MAP_ANONYMOUS)) {
 		audit_mmap_fd(fd, flags);
 		file = fget(fd);
diff --git a/mm/mremap.c b/mm/mremap.c
index 122938dcec15..af363063ea23 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -607,7 +607,6 @@ SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len,
 	LIST_HEAD(uf_unmap);
 
 	addr = untagged_addr(addr);
-	new_addr = untagged_addr(new_addr);
 
 	if (flags & ~(MREMAP_FIXED | MREMAP_MAYMOVE))
 		return ret;

Re: [PATCH] mm: Avoid creating virtual address aliases in brk()/mmap()/mremap()

[Will Deacon]

On Tue, Feb 18, 2020 at 12:23:10PM +0000, Catalin Marinas wrote:
> Currently the arm64 kernel ignores the top address byte passed to brk(),
> mmap() and mremap(). When the user is not aware of the 56-bit address
> limit or relies on the kernel to return an error, untagging such
> pointers has the potential to create address aliases in user-space.
> Passing a tagged address to munmap(), madvise() is permitted since the
> tagged pointer is expected to be inside an existing mapping.

Might be worth mentioning that this is causing real issues for existing
userspace:

https://bugzilla.redhat.com/show_bug.cgi?id=1797052

and so should be merged as a fix.

> Remove untagging in the above functions by partially reverting commit
> ce18d171cb73 ("mm: untag user pointers in mmap/munmap/mremap/brk"). In
> addition, update the arm64 tagged-address-abi.rst document accordingly.
> 
> Fixes: ce18d171cb73 ("mm: untag user pointers in mmap/munmap/mremap/brk")
> Cc: <stable@vger.kernel.org> # 5.4.x-
> Cc: Andrey Konovalov <andreyknvl@google.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Florian Weimer <fweimer@redhat.com>
> Reported-by: Victor Stinner <vstinner@redhat.com>
> Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
> ---
>  Documentation/arm64/tagged-address-abi.rst | 7 +++++--
>  mm/mmap.c                                  | 4 ----
>  mm/mremap.c                                | 1 -
>  3 files changed, 5 insertions(+), 7 deletions(-)
> 
> diff --git a/Documentation/arm64/tagged-address-abi.rst b/Documentation/arm64/tagged-address-abi.rst
> index d4a85d535bf9..1771a8b5712e 100644
> --- a/Documentation/arm64/tagged-address-abi.rst
> +++ b/Documentation/arm64/tagged-address-abi.rst
> @@ -44,8 +44,11 @@ The AArch64 Tagged Address ABI has two stages of relaxation depending
>  how the user addresses are used by the kernel:
>  
>  1. User addresses not accessed by the kernel but used for address space
> -   management (e.g. ``mmap()``, ``mprotect()``, ``madvise()``). The use
> -   of valid tagged pointers in this context is always allowed.
> +   management (e.g. ``mprotect()``, ``madvise()``). The use of valid
> +   tagged pointers in this context is allowed with the exception of
> +   ``brk()``, ``mmap()`` and the ``new_address`` argument to
> +   ``mremap()`` as these have the potential of aliasing with existing
> +   user addresses.

Given that we're backporting this to stable kernels, perhaps it's worth
a note here along the lines of:

NOTE: This behaviour changed in v5.6 and so some earlier kernels may
incorrectly accept valid tagged pointers for these system calls.

With that:

Acked-by: Will Deacon <will@kernel.org>

Happy to take this as an arm64 fix for 5.6, unless Andrew would prefer
to route it via his tree.

Will

Re: [PATCH] mm: Avoid creating virtual address aliases in brk()/mmap()/mremap()

[Andrey Konovalov]

On Tue, Feb 18, 2020 at 1:34 PM Will Deacon <will@kernel.org> wrote:
>
> On Tue, Feb 18, 2020 at 12:23:10PM +0000, Catalin Marinas wrote:
> > Currently the arm64 kernel ignores the top address byte passed to brk(),
> > mmap() and mremap(). When the user is not aware of the 56-bit address
> > limit or relies on the kernel to return an error, untagging such
> > pointers has the potential to create address aliases in user-space.
> > Passing a tagged address to munmap(), madvise() is permitted since the
> > tagged pointer is expected to be inside an existing mapping.
>
> Might be worth mentioning that this is causing real issues for existing
> userspace:
>
> https://bugzilla.redhat.com/show_bug.cgi?id=1797052
>
> and so should be merged as a fix.
>
> > Remove untagging in the above functions by partially reverting commit
> > ce18d171cb73 ("mm: untag user pointers in mmap/munmap/mremap/brk"). In
> > addition, update the arm64 tagged-address-abi.rst document accordingly.
> >
> > Fixes: ce18d171cb73 ("mm: untag user pointers in mmap/munmap/mremap/brk")
> > Cc: <stable@vger.kernel.org> # 5.4.x-
> > Cc: Andrey Konovalov <andreyknvl@google.com>
> > Cc: Will Deacon <will@kernel.org>
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: Florian Weimer <fweimer@redhat.com>
> > Reported-by: Victor Stinner <vstinner@redhat.com>
> > Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
> > ---
> >  Documentation/arm64/tagged-address-abi.rst | 7 +++++--
> >  mm/mmap.c                                  | 4 ----
> >  mm/mremap.c                                | 1 -
> >  3 files changed, 5 insertions(+), 7 deletions(-)
> >
> > diff --git a/Documentation/arm64/tagged-address-abi.rst b/Documentation/arm64/tagged-address-abi.rst
> > index d4a85d535bf9..1771a8b5712e 100644
> > --- a/Documentation/arm64/tagged-address-abi.rst
> > +++ b/Documentation/arm64/tagged-address-abi.rst
> > @@ -44,8 +44,11 @@ The AArch64 Tagged Address ABI has two stages of relaxation depending
> >  how the user addresses are used by the kernel:
> >
> >  1. User addresses not accessed by the kernel but used for address space
> > -   management (e.g. ``mmap()``, ``mprotect()``, ``madvise()``). The use
> > -   of valid tagged pointers in this context is always allowed.
> > +   management (e.g. ``mprotect()``, ``madvise()``). The use of valid
> > +   tagged pointers in this context is allowed with the exception of
> > +   ``brk()``, ``mmap()`` and the ``new_address`` argument to
> > +   ``mremap()`` as these have the potential of aliasing with existing
> > +   user addresses.
>
> Given that we're backporting this to stable kernels, perhaps it's worth
> a note here along the lines of:
>
> NOTE: This behaviour changed in v5.6 and so some earlier kernels may
> incorrectly accept valid tagged pointers for these system calls.
>
> With that:
>
> Acked-by: Will Deacon <will@kernel.org>
>
> Happy to take this as an arm64 fix for 5.6, unless Andrew would prefer
> to route it via his tree.
>
> Will

Re: [PATCH] mm: Avoid creating virtual address aliases in brk()/mmap()/mremap()

[Andrey Konovalov]

On Tue, Feb 18, 2020 at 1:34 PM Will Deacon <will@kernel.org> wrote:
>
> On Tue, Feb 18, 2020 at 12:23:10PM +0000, Catalin Marinas wrote:
> > Currently the arm64 kernel ignores the top address byte passed to brk(),
> > mmap() and mremap(). When the user is not aware of the 56-bit address
> > limit or relies on the kernel to return an error, untagging such
> > pointers has the potential to create address aliases in user-space.
> > Passing a tagged address to munmap(), madvise() is permitted since the
> > tagged pointer is expected to be inside an existing mapping.
>
> Might be worth mentioning that this is causing real issues for existing
> userspace:
>
> https://bugzilla.redhat.com/show_bug.cgi?id=1797052
>
> and so should be merged as a fix.
>
> > Remove untagging in the above functions by partially reverting commit
> > ce18d171cb73 ("mm: untag user pointers in mmap/munmap/mremap/brk"). In
> > addition, update the arm64 tagged-address-abi.rst document accordingly.

Evgenii, do you know if this will cause any issues for HWASAN?

> >
> > Fixes: ce18d171cb73 ("mm: untag user pointers in mmap/munmap/mremap/brk")
> > Cc: <stable@vger.kernel.org> # 5.4.x-
> > Cc: Andrey Konovalov <andreyknvl@google.com>
> > Cc: Will Deacon <will@kernel.org>
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: Florian Weimer <fweimer@redhat.com>
> > Reported-by: Victor Stinner <vstinner@redhat.com>
> > Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
> > ---
> >  Documentation/arm64/tagged-address-abi.rst | 7 +++++--
> >  mm/mmap.c                                  | 4 ----
> >  mm/mremap.c                                | 1 -
> >  3 files changed, 5 insertions(+), 7 deletions(-)
> >
> > diff --git a/Documentation/arm64/tagged-address-abi.rst b/Documentation/arm64/tagged-address-abi.rst
> > index d4a85d535bf9..1771a8b5712e 100644
> > --- a/Documentation/arm64/tagged-address-abi.rst
> > +++ b/Documentation/arm64/tagged-address-abi.rst
> > @@ -44,8 +44,11 @@ The AArch64 Tagged Address ABI has two stages of relaxation depending
> >  how the user addresses are used by the kernel:
> >
> >  1. User addresses not accessed by the kernel but used for address space
> > -   management (e.g. ``mmap()``, ``mprotect()``, ``madvise()``). The use
> > -   of valid tagged pointers in this context is always allowed.
> > +   management (e.g. ``mprotect()``, ``madvise()``). The use of valid
> > +   tagged pointers in this context is allowed with the exception of
> > +   ``brk()``, ``mmap()`` and the ``new_address`` argument to
> > +   ``mremap()`` as these have the potential of aliasing with existing
> > +   user addresses.
>
> Given that we're backporting this to stable kernels, perhaps it's worth
> a note here along the lines of:
>
> NOTE: This behaviour changed in v5.6 and so some earlier kernels may
> incorrectly accept valid tagged pointers for these system calls.
>
> With that:
>
> Acked-by: Will Deacon <will@kernel.org>
>
> Happy to take this as an arm64 fix for 5.6, unless Andrew would prefer
> to route it via his tree.
>
> Will

Re: [PATCH] mm: Avoid creating virtual address aliases in brk()/mmap()/mremap()

[Evgenii Stepanov]

On Tue, Feb 18, 2020 at 5:07 AM Andrey Konovalov <andreyknvl@google.com> wrote:
>
> On Tue, Feb 18, 2020 at 1:34 PM Will Deacon <will@kernel.org> wrote:
> >
> > On Tue, Feb 18, 2020 at 12:23:10PM +0000, Catalin Marinas wrote:
> > > Currently the arm64 kernel ignores the top address byte passed to brk(),
> > > mmap() and mremap(). When the user is not aware of the 56-bit address
> > > limit or relies on the kernel to return an error, untagging such
> > > pointers has the potential to create address aliases in user-space.
> > > Passing a tagged address to munmap(), madvise() is permitted since the
> > > tagged pointer is expected to be inside an existing mapping.
> >
> > Might be worth mentioning that this is causing real issues for existing
> > userspace:
> >
> > https://bugzilla.redhat.com/show_bug.cgi?id=1797052
> >
> > and so should be merged as a fix.
> >
> > > Remove untagging in the above functions by partially reverting commit
> > > ce18d171cb73 ("mm: untag user pointers in mmap/munmap/mremap/brk"). In
> > > addition, update the arm64 tagged-address-abi.rst document accordingly.
>
> Evgenii, do you know if this will cause any issues for HWASAN?

Is it possible to preserve the untagging behavior when a process has
opted in TBI?

I have not seen an actual issue with a tagged pointer in mmap yet
(I've seen two with mprotect, but not mmap or sbrk), so we should be
fine either way.

>
> > >
> > > Fixes: ce18d171cb73 ("mm: untag user pointers in mmap/munmap/mremap/brk")
> > > Cc: <stable@vger.kernel.org> # 5.4.x-
> > > Cc: Andrey Konovalov <andreyknvl@google.com>
> > > Cc: Will Deacon <will@kernel.org>
> > > Cc: Andrew Morton <akpm@linux-foundation.org>
> > > Cc: Florian Weimer <fweimer@redhat.com>
> > > Reported-by: Victor Stinner <vstinner@redhat.com>
> > > Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
> > > ---
> > >  Documentation/arm64/tagged-address-abi.rst | 7 +++++--
> > >  mm/mmap.c                                  | 4 ----
> > >  mm/mremap.c                                | 1 -
> > >  3 files changed, 5 insertions(+), 7 deletions(-)
> > >
> > > diff --git a/Documentation/arm64/tagged-address-abi.rst b/Documentation/arm64/tagged-address-abi.rst
> > > index d4a85d535bf9..1771a8b5712e 100644
> > > --- a/Documentation/arm64/tagged-address-abi.rst
> > > +++ b/Documentation/arm64/tagged-address-abi.rst
> > > @@ -44,8 +44,11 @@ The AArch64 Tagged Address ABI has two stages of relaxation depending
> > >  how the user addresses are used by the kernel:
> > >
> > >  1. User addresses not accessed by the kernel but used for address space
> > > -   management (e.g. ``mmap()``, ``mprotect()``, ``madvise()``). The use
> > > -   of valid tagged pointers in this context is always allowed.
> > > +   management (e.g. ``mprotect()``, ``madvise()``). The use of valid
> > > +   tagged pointers in this context is allowed with the exception of
> > > +   ``brk()``, ``mmap()`` and the ``new_address`` argument to
> > > +   ``mremap()`` as these have the potential of aliasing with existing
> > > +   user addresses.
> >
> > Given that we're backporting this to stable kernels, perhaps it's worth
> > a note here along the lines of:
> >
> > NOTE: This behaviour changed in v5.6 and so some earlier kernels may
> > incorrectly accept valid tagged pointers for these system calls.
> >
> > With that:
> >
> > Acked-by: Will Deacon <will@kernel.org>
> >
> > Happy to take this as an arm64 fix for 5.6, unless Andrew would prefer
> > to route it via his tree.
> >
> > Will

Re: [PATCH] mm: Avoid creating virtual address aliases in brk()/mmap()/mremap()

[Will Deacon]

On Tue, Feb 18, 2020 at 01:05:14PM -0800, Evgenii Stepanov wrote:
> On Tue, Feb 18, 2020 at 5:07 AM Andrey Konovalov <andreyknvl@google.com> wrote:
> >
> > On Tue, Feb 18, 2020 at 1:34 PM Will Deacon <will@kernel.org> wrote:
> > >
> > > On Tue, Feb 18, 2020 at 12:23:10PM +0000, Catalin Marinas wrote:
> > > > Currently the arm64 kernel ignores the top address byte passed to brk(),
> > > > mmap() and mremap(). When the user is not aware of the 56-bit address
> > > > limit or relies on the kernel to return an error, untagging such
> > > > pointers has the potential to create address aliases in user-space.
> > > > Passing a tagged address to munmap(), madvise() is permitted since the
> > > > tagged pointer is expected to be inside an existing mapping.
> > >
> > > Might be worth mentioning that this is causing real issues for existing
> > > userspace:
> > >
> > > https://bugzilla.redhat.com/show_bug.cgi?id=1797052
> > >
> > > and so should be merged as a fix.
> > >
> > > > Remove untagging in the above functions by partially reverting commit
> > > > ce18d171cb73 ("mm: untag user pointers in mmap/munmap/mremap/brk"). In
> > > > addition, update the arm64 tagged-address-abi.rst document accordingly.
> >
> > Evgenii, do you know if this will cause any issues for HWASAN?
> 
> Is it possible to preserve the untagging behavior when a process has
> opted in TBI?

It's /possible/, but without a concrete need I'm not keen to special case
mmap() like this. "Avoid aliasing user addresses" is an enforceable rule
across all system calls and is easily documented as such, so I'd prefer
to start from that position and only add exceptions where they are really
needed. That clearly doesn't preclude adding them later on with an extension
to the current prctl() controls.

> I have not seen an actual issue with a tagged pointer in mmap yet
> (I've seen two with mprotect, but not mmap or sbrk), so we should be
> fine either way.

Right, and we're leaving mprotect() and madvise() as they were.

Will

Re: [PATCH] mm: Avoid creating virtual address aliases in brk()/mmap()/mremap()

[Andrey Konovalov]

On Tue, Feb 18, 2020 at 1:23 PM Catalin Marinas <catalin.marinas@arm.com> wrote:
>
> Currently the arm64 kernel ignores the top address byte passed to brk(),
> mmap() and mremap(). When the user is not aware of the 56-bit address
> limit or relies on the kernel to return an error, untagging such
> pointers has the potential to create address aliases in user-space.
> Passing a tagged address to munmap(), madvise() is permitted since the
> tagged pointer is expected to be inside an existing mapping.
>
> Remove untagging in the above functions by partially reverting commit
> ce18d171cb73 ("mm: untag user pointers in mmap/munmap/mremap/brk"). In
> addition, update the arm64 tagged-address-abi.rst document accordingly.
>
> Fixes: ce18d171cb73 ("mm: untag user pointers in mmap/munmap/mremap/brk")
> Cc: <stable@vger.kernel.org> # 5.4.x-
> Cc: Andrey Konovalov <andreyknvl@google.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Florian Weimer <fweimer@redhat.com>
> Reported-by: Victor Stinner <vstinner@redhat.com>
> Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

Acked-by: Andrey Konovalov <andreyknvl@google.com>

> ---
>  Documentation/arm64/tagged-address-abi.rst | 7 +++++--
>  mm/mmap.c                                  | 4 ----
>  mm/mremap.c                                | 1 -
>  3 files changed, 5 insertions(+), 7 deletions(-)
>
> diff --git a/Documentation/arm64/tagged-address-abi.rst b/Documentation/arm64/tagged-address-abi.rst
> index d4a85d535bf9..1771a8b5712e 100644
> --- a/Documentation/arm64/tagged-address-abi.rst
> +++ b/Documentation/arm64/tagged-address-abi.rst
> @@ -44,8 +44,11 @@ The AArch64 Tagged Address ABI has two stages of relaxation depending
>  how the user addresses are used by the kernel:
>
>  1. User addresses not accessed by the kernel but used for address space
> -   management (e.g. ``mmap()``, ``mprotect()``, ``madvise()``). The use
> -   of valid tagged pointers in this context is always allowed.
> +   management (e.g. ``mprotect()``, ``madvise()``). The use of valid
> +   tagged pointers in this context is allowed with the exception of
> +   ``brk()``, ``mmap()`` and the ``new_address`` argument to
> +   ``mremap()`` as these have the potential of aliasing with existing
> +   user addresses.
>
>  2. User addresses accessed by the kernel (e.g. ``write()``). This ABI
>     relaxation is disabled by default and the application thread needs to
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 6756b8bb0033..d681a20eb4ea 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -195,8 +195,6 @@ SYSCALL_DEFINE1(brk, unsigned long, brk)
>         bool downgraded = false;
>         LIST_HEAD(uf);
>
> -       brk = untagged_addr(brk);
> -
>         if (down_write_killable(&mm->mmap_sem))
>                 return -EINTR;
>
> @@ -1557,8 +1555,6 @@ unsigned long ksys_mmap_pgoff(unsigned long addr, unsigned long len,
>         struct file *file = NULL;
>         unsigned long retval;
>
> -       addr = untagged_addr(addr);
> -
>         if (!(flags & MAP_ANONYMOUS)) {
>                 audit_mmap_fd(fd, flags);
>                 file = fget(fd);
> diff --git a/mm/mremap.c b/mm/mremap.c
> index 122938dcec15..af363063ea23 100644
> --- a/mm/mremap.c
> +++ b/mm/mremap.c
> @@ -607,7 +607,6 @@ SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len,
>         LIST_HEAD(uf_unmap);
>
>         addr = untagged_addr(addr);
> -       new_addr = untagged_addr(new_addr);
>
>         if (flags & ~(MREMAP_FIXED | MREMAP_MAYMOVE))
>                 return ret;