Message-ID: <2979df31-ce8c-4382-ab01-7e66f852099d@suse.cz>
Date: Fri, 15 Nov 2024 20:06:05 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, stable@...r.kernel.org
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 "Liam R . Howlett" <Liam.Howlett@...cle.com>, Jann Horn <jannh@...gle.com>,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 Linus Torvalds <torvalds@...ux-foundation.org>, Peter Xu
 <peterx@...hat.com>, Catalin Marinas <catalin.marinas@....com>,
 Will Deacon <will@...nel.org>, Mark Brown <broonie@...nel.org>,
 "David S . Miller" <davem@...emloft.net>,
 Andreas Larsson <andreas@...sler.com>,
 "James E . J . Bottomley" <James.Bottomley@...senPartnership.com>,
 Helge Deller <deller@....de>
Subject: Re: [PATCH 6.1.y 4/4] mm: resolve faulty mmap_region() error path
 behaviour

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@oracle.com
> Fixes: deb0f6562884 ("mm/mmap: undo ->mmap() when arch_validate_flags() fails")
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> Reported-by: Jann Horn <jannh@...gle.com>
> Reviewed-by: Liam R. Howlett <Liam.Howlett@...cle.com>
> Reviewed-by: Vlastimil Babka <vbabka@...e.cz>
> Tested-by: Mark Brown <broonie@...nel.org>
> Cc: Andreas Larsson <andreas@...sler.com>
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: David S. Miller <davem@...emloft.net>
> Cc: Helge Deller <deller@....de>
> Cc: James E.J. Bottomley <James.Bottomley@...senPartnership.com>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: Peter Xu <peterx@...hat.com>
> Cc: Will Deacon <will@...nel.org>
> Cc: <stable@...r.kernel.org>
> Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> ---
>  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;
> -				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?

>  			}
>  		}
>  
> @@ -2800,31 +2802,15 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
>  	} else if (vm_flags & VM_SHARED) {
>  		error = shmem_zero_setup(vma);
>  		if (error)
> -			goto free_vma;
> +			goto free_iter_vma;
>  	} else {
>  		vma_set_anonymous(vma);
>  	}
>  
> -	/* Allow architectures to sanity-check the vm_flags */
> -	if (!arch_validate_flags(vma->vm_flags)) {
> -		error = -EINVAL;
> -		if (file)
> -			goto close_and_free_vma;
> -		else if (vma->vm_file)
> -			goto unmap_and_free_vma;
> -		else
> -			goto free_vma;
> -	}
> -
> -	if (mas_preallocate(&mas, vma, GFP_KERNEL)) {
> -		error = -ENOMEM;
> -		if (file)
> -			goto close_and_free_vma;
> -		else if (vma->vm_file)
> -			goto unmap_and_free_vma;
> -		else
> -			goto free_vma;
> -	}
> +#ifdef CONFIG_SPARC64
> +	/* TODO: Fix SPARC ADI! */
> +	WARN_ON_ONCE(!arch_validate_flags(vm_flags));
> +#endif
>  
>  	if (vma->vm_file)
>  		i_mmap_lock_write(vma->vm_file->f_mapping);
> @@ -2847,10 +2833,7 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
>  	 */
>  	khugepaged_enter_vma(vma, vma->vm_flags);
>  
> -	/* Once vma denies write, undo our temporary denial count */
> -unmap_writable:
> -	if (file && vm_flags & VM_SHARED)
> -		mapping_unmap_writable(file->f_mapping);
> +file_expanded:
>  	file = vma->vm_file;
>  expanded:
>  	perf_event_mmap(vma);
> @@ -2879,28 +2862,54 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
>  
>  	vma_set_page_prot(vma);
>  
> -	validate_mm(mm);
>  	return addr;
>  
> -close_and_free_vma:
> -	vma_close(vma);
> -unmap_and_free_vma:
> +unmap_and_free_file_vma:
>  	fput(vma->vm_file);
>  	vma->vm_file = NULL;
>  
>  	/* Undo any partial mapping done by a device driver. */
>  	unmap_region(mm, mas.tree, vma, prev, next, vma->vm_start, vma->vm_end);
> -	if (file && (vm_flags & VM_SHARED))
> -		mapping_unmap_writable(file->f_mapping);
> +free_iter_vma:
> +	mas_destroy(&mas);
>  free_vma:
>  	vm_area_free(vma);
>  unacct_error:
>  	if (charged)
>  		vm_unacct_memory(charged);
> -	validate_mm(mm);
>  	return error;
>  }
>  
> +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)
> +{
> +	unsigned long ret;
> +	bool writable_file_mapping = false;
> +
> +	/* Allow architectures to sanity-check the vm_flags. */
> +	if (!arch_validate_flags(vm_flags))
> +		return -EINVAL;
> +
> +	/* Map writable and ensure this isn't a sealed memfd. */
> +	if (file && (vm_flags & VM_SHARED)) {
> +		int error = mapping_map_writable(file->f_mapping);
> +
> +		if (error)
> +			return error;
> +		writable_file_mapping = true;
> +	}
> +
> +	ret = __mmap_region(file, addr, len, vm_flags, pgoff, uf);
> +
> +	/* Clear our write mapping regardless of error. */
> +	if (writable_file_mapping)
> +		mapping_unmap_writable(file->f_mapping);
> +
> +	validate_mm(current->mm);
> +	return ret;
> +}
> +
>  static int __vm_munmap(unsigned long start, size_t len, bool downgrade)
>  {
>  	int ret;

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