Re: [PATCH v3 1/2] Ext4Pkg: Improve extent node validation on the number of entries

["Pedro Falcato" <pedro.falcato@gmail.com>]

Thank you! Merged the patchset as 82f4b39 and 9247980, respectively.

On Tue, May 9, 2023 at 5:07 PM Marvin Häuser <mhaeuser@posteo.de> wrote:
>
> Reviewed-by: Marvin Häuser <mhaeuser@posteo.de>
>
> > On 9. May 2023, at 17:49, 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 | 32 ++++++++++++++++++++++++++----
> > 1 file changed, 28 insertions(+), 4 deletions(-)
> >
> > diff --git a/Features/Ext4Pkg/Ext4Dxe/Extents.c b/Features/Ext4Pkg/Ext4Dxe/Extents.c
> > index 9e4364e50d99..2b82417c9e10 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;
> >   }
> >
> > +  // Note: We do not need to check eh_entries here, as the next branch makes sure max >= entries
> > +  if (Header->eh_max > MaxEntries) {
> > +    DEBUG ((
> > +      DEBUG_ERROR,
> > +      "[ext4] Invalid extent header max entries (%u eh_max, "
> > +      "theoretical max is %u) (larger than permitted)\n",
> > +      Header->eh_max,
> > +      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.
> > @@ -237,6 +254,7 @@ Ext4GetExtent (
> >   EXT4_EXTENT_HEADER  *ExtHeader;
> >   EXT4_EXTENT_INDEX   *Index;
> >   EFI_STATUS          Status;
> > +  UINT32              MaxExtentsPerNode;
> >   EXT4_BLOCK_NR       BlockNumber;
> >
> >   Inode  = File->Inode;
> > @@ -275,12 +293,17 @@ 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
> > @@ -309,6 +332,7 @@ Ext4GetExtent (
> >     }
> >
> >     // Read the leaf block onto the previously-allocated buffer.
> > +
> >     Status = Ext4ReadBlocks (Partition, Buffer, 1, BlockNumber);
> >     if (EFI_ERROR (Status)) {
> >       FreePool (Buffer);
> > @@ -317,7 +341,7 @@ Ext4GetExtent (
> >
> >     ExtHeader = Buffer;
> >
> > -    if (!Ext4ExtentHeaderValid (ExtHeader)) {
> > +    if (!Ext4ExtentHeaderValid (ExtHeader, MaxExtentsPerNode)) {
> >       FreePool (Buffer);
> >       return EFI_VOLUME_CORRUPTED;
> >     }
> > --
> > 2.40.1
> >
>


-- 
Pedro