* Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx> [241115 14:28]: > On Fri, Nov 15, 2024 at 08:06:05PM +0100, Vlastimil Babka wrote: > > On 11/15/24 13:40, Lorenzo Stoakes wrote: > > > [ Upstream commit 5de195060b2e251a835f622759550e6202167641 ] > > > > > > The mmap_region() function is somewhat terrifying, with spaghetti-like > > > control flow and numerous means by which issues can arise and incomplete > > > state, memory leaks and other unpleasantness can occur. > > > > > > A large amount of the complexity arises from trying to handle errors late > > > in the process of mapping a VMA, which forms the basis of recently > > > observed issues with resource leaks and observable inconsistent state. > > > > > > Taking advantage of previous patches in this series we move a number of > > > checks earlier in the code, simplifying things by moving the core of the > > > logic into a static internal function __mmap_region(). > > > > > > Doing this allows us to perform a number of checks up front before we do > > > any real work, and allows us to unwind the writable unmap check > > > unconditionally as required and to perform a CONFIG_DEBUG_VM_MAPLE_TREE > > > validation unconditionally also. > > > > > > We move a number of things here: > > > > > > 1. We preallocate memory for the iterator before we call the file-backed > > > memory hook, allowing us to exit early and avoid having to perform > > > complicated and error-prone close/free logic. We carefully free > > > iterator state on both success and error paths. > > > > > > 2. The enclosing mmap_region() function handles the mapping_map_writable() > > > logic early. Previously the logic had the mapping_map_writable() at the > > > point of mapping a newly allocated file-backed VMA, and a matching > > > mapping_unmap_writable() on success and error paths. > > > > > > We now do this unconditionally if this is a file-backed, shared writable > > > mapping. If a driver changes the flags to eliminate VM_MAYWRITE, however > > > doing so does not invalidate the seal check we just performed, and we in > > > any case always decrement the counter in the wrapper. > > > > > > We perform a debug assert to ensure a driver does not attempt to do the > > > opposite. > > > > > > 3. We also move arch_validate_flags() up into the mmap_region() > > > function. This is only relevant on arm64 and sparc64, and the check is > > > only meaningful for SPARC with ADI enabled. We explicitly add a warning > > > for this arch if a driver invalidates this check, though the code ought > > > eventually to be fixed to eliminate the need for this. > > > > > > With all of these measures in place, we no longer need to explicitly close > > > the VMA on error paths, as we place all checks which might fail prior to a > > > call to any driver mmap hook. > > > > > > This eliminates an entire class of errors, makes the code easier to reason > > > about and more robust. > > > > > > Link: https://lkml.kernel.org/r/6e0becb36d2f5472053ac5d544c0edfe9b899e25.1730224667.git.lorenzo.stoakes@xxxxxxxxxx > > > Fixes: deb0f6562884 ("mm/mmap: undo ->mmap() when arch_validate_flags() fails") > > > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx> > > > Reported-by: Jann Horn <jannh@xxxxxxxxxx> > > > Reviewed-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx> > > > Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx> > > > Tested-by: Mark Brown <broonie@xxxxxxxxxx> > > > Cc: Andreas Larsson <andreas@xxxxxxxxxxx> > > > Cc: Catalin Marinas <catalin.marinas@xxxxxxx> > > > Cc: David S. Miller <davem@xxxxxxxxxxxxx> > > > Cc: Helge Deller <deller@xxxxxx> > > > Cc: James E.J. Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx> > > > Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> > > > Cc: Peter Xu <peterx@xxxxxxxxxx> > > > Cc: Will Deacon <will@xxxxxxxxxx> > > > Cc: <stable@xxxxxxxxxxxxxxx> > > > Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> > > > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx> > > > --- > > > mm/mmap.c | 103 +++++++++++++++++++++++++++++------------------------- > > > 1 file changed, 56 insertions(+), 47 deletions(-) > > > > > > diff --git a/mm/mmap.c b/mm/mmap.c > > > index 322677f61d30..e457169c5cce 100644 > > > --- a/mm/mmap.c > > > +++ b/mm/mmap.c > > > @@ -2652,7 +2652,7 @@ int do_munmap(struct mm_struct *mm, unsigned long start, size_t len, > > > return do_mas_munmap(&mas, mm, start, len, uf, false); > > > } > > > > > > -unsigned long mmap_region(struct file *file, unsigned long addr, > > > +static unsigned long __mmap_region(struct file *file, unsigned long addr, > > > unsigned long len, vm_flags_t vm_flags, unsigned long pgoff, > > > struct list_head *uf) > > > { > > > @@ -2750,26 +2750,28 @@ unsigned long mmap_region(struct file *file, unsigned long addr, > > > vma->vm_page_prot = vm_get_page_prot(vm_flags); > > > vma->vm_pgoff = pgoff; > > > > > > - if (file) { > > > - if (vm_flags & VM_SHARED) { > > > - error = mapping_map_writable(file->f_mapping); > > > - if (error) > > > - goto free_vma; > > > - } > > > + if (mas_preallocate(&mas, vma, GFP_KERNEL)) { > > > + error = -ENOMEM; > > > + goto free_vma; > > > + } > > > > > > + if (file) { > > > vma->vm_file = get_file(file); > > > error = mmap_file(file, vma); > > > if (error) > > > - goto unmap_and_free_vma; > > > + goto unmap_and_free_file_vma; > > > + > > > + /* Drivers cannot alter the address of the VMA. */ > > > + WARN_ON_ONCE(addr != vma->vm_start); > > > > > > /* > > > - * Expansion is handled above, merging is handled below. > > > - * Drivers should not alter the address of the VMA. > > > + * Drivers should not permit writability when previously it was > > > + * disallowed. > > > */ > > > - if (WARN_ON((addr != vma->vm_start))) { > > > - error = -EINVAL; > > > - goto close_and_free_vma; > > > - } > > > + VM_WARN_ON_ONCE(vm_flags != vma->vm_flags && > > > + !(vm_flags & VM_MAYWRITE) && > > > + (vma->vm_flags & VM_MAYWRITE)); > > > + > > > mas_reset(&mas); > > > > > > /* > > > @@ -2792,7 +2794,7 @@ unsigned long mmap_region(struct file *file, unsigned long addr, > > > vma = merge; > > > /* Update vm_flags to pick up the change. */ > > > vm_flags = vma->vm_flags; > > As far as I can tell we should add: > > + mas_destroy(&mas); Yes, this will fix it. > > > > - goto unmap_writable; > > > + goto file_expanded; > > > > I think we might need a mas_destroy() somewhere around here otherwise we > > leak the prealloc? In later versions the merge operation takes our vma > > iterator so it handles that if merge succeeds, but here we have to cleanup > > our mas ourselves? > > > > Sigh, yup. This code path is SO HORRIBLE. I think simply a > mas_destroy(&mas) here would suffice (see above). Yes, clean up your own mas. Besides this change, the patch looks good. Thanks, Liam
