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["Guilherme G. Piccoli" <1874444@xxxxxxxxxxxxxxxxxx>]

** Changed in: linux (Ubuntu)
       Status: In Progress => Fix Released

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https://bugs.launchpad.net/bugs/1874444

Title:
  Bionic ubuntu ethtool doesn't check ring parameters boundaries

Status in linux package in Ubuntu:
  Fix Released
Status in linux source package in Xenial:
  Fix Released
Status in linux source package in Bionic:
  Fix Released

Bug description:
  [Impact]
  * There's a bad behavior in the ena driver ringparam setting on kernels 4.4 and 4.15, if an invalid ringparam is provided to ethtool.

  * Upstream Linux kernel implemented ring parameter boundaries check in commit: 37e2d99b59c4 ("ethtool: Ensure new ring parameters are within bounds during SRINGPARAM") [ git.kernel.org/linus/37e2d99b59c4 ].
  Due to this commit, the community doesn't usually allow ring parameter boundary checks in driver code.

  * Xenial/Bionic kernels don't include this patch, and some network
  drivers (like ena) rely on this patch for boundary checking of ring
  params. So, we are hereby requesting the commit inclusion in these
  kernel versions.

  [Test case]
  1. In AWS, create a new c5.4xlarge instance with the Ubuntu 18.04 official ami (uses the ENA network driver) and update to latest kernel/reboot.

  2. Run ethtool -g ens5
  output:
  Ring parameters for ens5:
  Pre-set maximums:
  RX:		16384
  RX Mini:	0
  RX Jumbo:	0
  TX:		1024
  Current hardware settings:
  RX:		1024
  RX Mini:	0
  RX Jumbo:	0
  TX:		1024

  3. Change the TX/RX ring size to a legal number within boundaries -
  works!

  4. Change the TX/RX ring size to an illegal number (such as 2048 for
  TX) with the command - "sudo ethtool -G ens5 tx 2048".

  Expected behavior - "Cannot set device ring parameters: Invalid argument"
  Actual behavior - causes a driver hang since boundaries are not checked by ethtool, effectively hanging the instance (given that AWS has no console to allow system manipulation).

  [Regression Potential]

  Since that the commit is present in kernels v4.16+ (including Ubuntu)
  and is quite small and self-contained, the regression risk is very
  reduced.

  One potential "regression" would be if some driver has bugs and
  provide bad values on get_ringparams, then the validation would be
  broken (allowing illegal values or refusing legal ones), but this
  wouldn't be a regression in the hereby proposed patch itself, it'd be
  only exposed by the patch.

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