Re: [PATCH 1/2] Ext4Pkg: Improve extent node validation on the number of entries

["Marvin Häuser" <mhaeuser@posteo.de>]

> On 9. May 2023, at 02:42, Pedro Falcato <pedro.falcato@gmail.com> wrote:
> 
> Improve the extent tree node validation by validating the number of
> entries the node advertises against the theoretical max (derived from
> the size of on-disk structs and the block size (or i_data, if inline
> extents).
> 
> Previously, we did not validate the number of entries. This could be
> exploited for out-of-bounds reads and crashes.
> 
> Cc: Marvin Häuser <mhaeuser@posteo.de>
> Fixes: d9ceedca6c8f ("Ext4Pkg: Add Ext4Dxe driver.")
> Reported-by: Savva Mitrofanov <savvamtr@gmail.com>
> Signed-off-by: Pedro Falcato <pedro.falcato@gmail.com>
> ---
> Features/Ext4Pkg/Ext4Dxe/Extents.c | 47 +++++++++++++++++++-----------
> 1 file changed, 30 insertions(+), 17 deletions(-)
> 
> diff --git a/Features/Ext4Pkg/Ext4Dxe/Extents.c b/Features/Ext4Pkg/Ext4Dxe/Extents.c
> index 9e4364e50d99..66d085938549 100644
> --- a/Features/Ext4Pkg/Ext4Dxe/Extents.c
> +++ b/Features/Ext4Pkg/Ext4Dxe/Extents.c
> @@ -1,7 +1,7 @@
> /** @file
>   Extent related routines
> 
> -  Copyright (c) 2021 - 2022 Pedro Falcato All rights reserved.
> +  Copyright (c) 2021 - 2023 Pedro Falcato All rights reserved.
>   SPDX-License-Identifier: BSD-2-Clause-Patent
> **/
> 
> @@ -80,13 +80,15 @@ Ext4GetInoExtentHeader (
> /**
>    Checks if an extent header is valid.
>    @param[in]      Header         Pointer to the EXT4_EXTENT_HEADER structure.
> +   @param[in]      MaxEntries     Maximum number of entries possible for this tree node.
> 
>    @return TRUE if valid, FALSE if not.
> **/
> STATIC
> BOOLEAN
> Ext4ExtentHeaderValid (
> -  IN CONST EXT4_EXTENT_HEADER  *Header
> +  IN CONST EXT4_EXTENT_HEADER  *Header,
> +  IN UINT16                    MaxEntries
>   )
> {
>   if (Header->eh_depth > EXT4_EXTENT_TREE_MAX_DEPTH) {
> @@ -99,6 +101,18 @@ Ext4ExtentHeaderValid (
>     return FALSE;
>   }
> 
> +  if ((Header->eh_max > MaxEntries) || (Header->eh_entries > MaxEntries)) {

For eh_entries, this is implicit via the check below.

> +    DEBUG ((
> +      DEBUG_ERROR,
> +      "[ext4] Invalid extent header entries (extent header: %u max,"
> +      " %u entries, theoretical max: %u) (larger than permitted)\n",
> +      Header->eh_max,
> +      Header->eh_entries,
> +      MaxEntries
> +      ));
> +    return FALSE;
> +  }
> +
>   if (Header->eh_max < Header->eh_entries) {
>     DEBUG ((
>       DEBUG_ERROR,
> @@ -212,6 +226,9 @@ Ext4ExtentIdxLeafBlock (
>   return LShiftU64 (Index->ei_leaf_hi, 32) | Index->ei_leaf_lo;
> }
> 
> +// Results of sizeof(i_data) / sizeof(extent) - 1 = 4
> +#define EXT4_NR_INLINE_EXTENTS  4
> +
> /**
>    Retrieves an extent from an EXT4 inode.
>    @param[in]      Partition     Pointer to the opened EXT4 partition.
> @@ -237,7 +254,7 @@ Ext4GetExtent (
>   EXT4_EXTENT_HEADER  *ExtHeader;
>   EXT4_EXTENT_INDEX   *Index;
>   EFI_STATUS          Status;
> -  EXT4_BLOCK_NR       BlockNumber;
> +  UINT32              MaxExtentsPerNode;
> 
>   Inode  = File->Inode;
>   Ext    = NULL;
> @@ -275,12 +292,17 @@ Ext4GetExtent (
> 
>   ExtHeader = Ext4GetInoExtentHeader (Inode);
> 
> -  if (!Ext4ExtentHeaderValid (ExtHeader)) {
> +  if (!Ext4ExtentHeaderValid (ExtHeader, EXT4_NR_INLINE_EXTENTS)) {
>     return EFI_VOLUME_CORRUPTED;
>   }
> 
>   CurrentDepth = ExtHeader->eh_depth;
> 
> +  // A single node fits into a single block, so we can only have (BlockSize / sizeof(EXT4_EXTENT)) - 1
> +  // extents in a single node. Note the -1, because both leaf and internal node headers are 12 bytes,
> +  // and so are individual entries.
> +  MaxExtentsPerNode = (Partition->BlockSize / sizeof (EXT4_EXTENT)) - 1;
> +
>   while (ExtHeader->eh_depth != 0) {
>     CurrentDepth--;
>     // While depth != 0, we're traversing the tree itself and not any leaves
> @@ -289,17 +311,7 @@ Ext4GetExtent (
>     // Therefore, we can use binary search, and it's actually the standard for doing so
>     // (see FreeBSD).
> 
> -    Index       = Ext4BinsearchExtentIndex (ExtHeader, LogicalBlock);
> -    BlockNumber = Ext4ExtentIdxLeafBlock (Index);
> -
> -    // Check that block isn't file hole
> -    if (BlockNumber == EXT4_BLOCK_FILE_HOLE)

Why is this no longer possible?

Best regards,
Marvin

> {
> -      if (Buffer != NULL) {
> -        FreePool (Buffer);
> -      }
> -
> -      return EFI_VOLUME_CORRUPTED;
> -    }
> +    Index = Ext4BinsearchExtentIndex (ExtHeader, LogicalBlock);
> 
>     if (Buffer == NULL) {
>       Buffer = AllocatePool (Partition->BlockSize);
> @@ -309,7 +321,8 @@ Ext4GetExtent (
>     }
> 
>     // Read the leaf block onto the previously-allocated buffer.
> -    Status = Ext4ReadBlocks (Partition, Buffer, 1, BlockNumber);
> +
> +    Status = Ext4ReadBlocks (Partition, Buffer, 1, Ext4ExtentIdxLeafBlock (Index));
>     if (EFI_ERROR (Status)) {
>       FreePool (Buffer);
>       return Status;
> @@ -317,7 +330,7 @@ Ext4GetExtent (
> 
>     ExtHeader = Buffer;
> 
> -    if (!Ext4ExtentHeaderValid (ExtHeader)) {
> +    if (!Ext4ExtentHeaderValid (ExtHeader, MaxExtentsPerNode)) {
>       FreePool (Buffer);
>       return EFI_VOLUME_CORRUPTED;
>     }
> -- 
> 2.40.1
>